Worried about how Congress will overhaul the tax code? Don't be.

Even the most astute Hill-watchers don't know how the final legislation will look -- or whether it will pass. So no one has an unfair advantage in calculating the tax consequences of investments this year.

Many provisions crucial to financial decisions will not be determined until some time in 1986, if Congress passes a tax bill at all. Nor does the evolution of tax overhaul so far provide many clues.

Consider the tax rate on capital gains as a case in point.

Under current law, 60 percent of the increase in value of a capital asset such as real estate or securities is not taxable when the asset is sold more than six months after it is acquired. In the 50 percent tax bracket, that means the top capital gains rate is 20 percent, or 50 percent of 40 percent.

The Treasury Department's initial tax-revision proposal, which was released a year ago, called for wiping out the capital-gains exclusion and instead indexing the value of capital assets to inflation.

President Reagan's version of the Treasury plan, released six months later, restored the exclusion, but set it at 50 percent. Combined with Reagan's proposed top rate of 35 percent, the top capital gains rate would have been 17.5 percent.

Into the fray in September marched House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.), whose tax overhaul proposal called for a 40 percent exclusion and thus, when combined with a top tax rate of 35 percent, a maximum capital gains rate of 21 percent.

Each one of those possibilities would produce a different rate of return on a particular investment and change their relative appeal compared to other investments. None of them is sure either of passage or rejection, and Congress could well select a fourth approach.

The Ways and Means Committee, which is in the throes of rewriting the tax code, has not yet taken up the issue of capital gains. Once members choose how they wish to tax gains, the House still must pass the legislation, the Senate Finance Committee must deliberate on tax revison and the full Senate -- not known for acting hastily -- will have to pass a bill. Even President Reagan's signature is not guaranteed.

"There is a lot of uncertainty there," said John M. Albertine, president of the American Business Conference, an association of high-growth companies.

Nonetheless, the broadest outlines of tax-overhaul legislation can be predicted by analysts willing to stick their necks out, and even vague forecasts can provide guidance for investors trying to arrange their affairs.

*Any tax bill Congress produces is almost sure to reduce personal tax rates for most taxpayers. The best guess now is that the top rate will end up more than 10 percentage points below its current level of 50 percent but slightly higher than the 35 percent proposed by Reagan.

Lower rates make some kinds of tax shelters less attractive than they would be at the current top rate of 50 percent. All deductions become less potent if, instead of saving 50 cents in taxes for every dollar deducted, the taxpayer saves 35 cents.

*Even if broad tax revision fails, Congress is almost sure to enact a stiff minimum tax on indviduals and corporations, perhaps as part of a deficit-reduction measure. The revision measure now before the Ways and Means Committee would impose a minimum tax with a rate of 25 percent, 5 percentage points higher than the current minimum tax.

That's more complicated than it sounds. The minimum tax works by requiring many high-income taxpayers to recalculate their taxes using fewer, and more limited, deductions, then paying 20 percent of their income as redefined (assuming that figure is larger than their tax would be under ordinary calculation). Rostenkowski's proposal would raise that rate to 25 percent and would expand the list of deductions whose use would be curtailed.

As a result, more taxpayers probably would be covered by the minimum tax. Like the current system, the Rostenkowski plan would exempt the first $40,000 of taxable income for married couples and $30,000 for singles. But David Green, tax manager for the Washington office of Touche Ross, says taxpayers will not have an easy way of knowing whether they should recalculate their taxes under the minimum-tax plan.

Which tax preferences will be included in the minimum-tax list won't be known unless and until Congress finishes its work on tax overhaul. Like restrictions on the deductions themselves, items on the list of preferences are the focus of heated lobbying.

The personal minimum tax also affects more than those who pay it. According to Bernard M. Shapiro, former staff chief of the Joint Committee on Taxation and now with Price Waterhouse, investors arrange their affairs so that they aren't covered by the minimum tax.

"It is a floor for the regular tax. People structure their investments so they're right above it. It's very significant. Every high-income investor knows exactly where the minimum tax is," Shapiro said.

Congress almost surely will increase the tax burden on business, especially on companies with relatively low tax rates. Those tax increases, in turn, could put some downward pressure on the price of affected companies' stock.

"I would be cautious about investing in firms with low tax rates -- defense, banks, General Electric, General Dynamics," Albertine said. "There will be a strong corporate minimum tax."

The same caveat could apply to manufacturing firms and others that make heavy use of two provisions considered likely to be restricted. For a tax bill to raise enough revenue to offset the cost of lower tax rates, it almost surely will have to curtail depreciation write-offs for investment in buildings and equipment, and it will have to wipe out the investment tax credit (ITC), which subsidizes up to 10 percent of the cost of new equipment. Companies with extensive foreign operations also might lose some of their tax advantages.

Conversely, industries that would be helped by a reduction in the corporate tax rate, such as retail and data-processing services, could gain a comparative advantage.

Rep. Donald J. Pease (D-Ohio) estimates that Ways and Means members will be permitted to compromise on the so-called capital formation provisions such as depreciation and the ITC, but only by enough to lose about $25 billion in tax revenue over five years. The probability that Rostenkowski will agree to full retention of the deduction for state and local taxes or to cut it back very little means that tax-writers will have little excess revenue to play with.

*For years, real estate has offered one of the juiciest tax-shelter opportunities around. Unlike such other shelters as oil and gas, real estate allows investors to take advantage of an exception to the so-called "at-risk" rule and write off more money than they have put in.

The at-risk rule is on the block, however. And the other major changes Congress is likely to make in depreciation, capital gains and the minimum tax also would combine to make real estate investments, especially in the form of limited partnerships, less advantageous.

Observers also offer two warnings about the list of deductions up for curtailment: One, Congress could well produce no tax bill at all, at least in the near term. And two, the markets may have discounted the possible changes to such an extent that the prices of many investments have fallen to reflect that risk. That means investing in sectors up for cutbacks -- vacation homes are an example -- still could make sense.

"What I would say to investors is, plan on business as usual," said economist John Makin of the American Enterprise Institute. "If you can find a seller of a tax shelter who will take a low price because he is nervous, buy it. Don't be poring over this tax bill as if something was going to happen in a serious way.