President Carter began a final round of decision-making yesterday on a set of tax proposals that includes sizeable cuts in withholding rates for individuals and outright abolition of $3 billion in air fare and telephone excise taxes, a step intended to counter inflation.

Carter discussed the outlines of the proposed legislation - and it's possible timing - in a series of meetings with his economic advisers and key members of Congress. The President later told White House aides he planned to make final decisions on most of the remaining issues last night and today.

Sources cautioned yeaterday that Carter still has not decided on many details of the package that was out-lined to congressional leaders, and some provision may be subject to change. However, the package included:

A two-part reduction in taxes for individuals - first by trimming with holding rates in the second part of 1978, than by ereducing actual tax rates in 1979.

Elimination in 1978 of the 4 percent excise tax on the telephone charges and the 8 per cent excise tax on air-plane fares, both as anti-inflation measures. Carter advisers say these would help hold prices down by reducing business and consumer costs, but congressional leaders reportedly are unethusiastic.

Replacement of familiar $750 personal tax exemption that every tax-payer can take for himself and each dependent with tax credit, a shift which would tend to benefit lower income persons. The switch to the tax credit of $240 or $250 per person would effectively cut taxes for most families earning $20,000 a year or less.

A special tax credit for working spouses help reduce the so-called "marriage penalty" that now results from the higher tax rates imposed on couples. Carter has been considering a 10 per cent credit last summer, but the final size had not beeen decided.

Elimination of the present federal income tax deductions for state sales taxes and gasoline taxes. Carter has said he wants to end these to help simplify the tax code. At the same time, the White House has scrapped proposals to trim back other deductions for individuals that it had been considering earlier.

Limiting the present deduction for interest paid on home mortgages to a maximum generally paid on a $100,000 mortgage. Administration of fiscals insists the limit would effect only a small portion of the nation's homeowners, but in the high-cost areas such as Washington the porpotion would run higher.

Some modest cutback in deductions for business expenses, such as so-called "martini lunches" - though not the 50 per cent limit that the administration had been considering in its September tax package. This proposal would also end company writeoffs for hunting lodges and other luxuries. (KEY OFF)(KEYWORD) two-percentage-point cut in business taxes, subtracting one point from the present 48 per cent corporate rate and one fron the so-called "small business surtax," the portion of the corporate tax that applies to the first $50,000 of a company's earnings.

Liberalization of the present 10 per cent tax credit for business investment - first by extending it to cover spending for structures as well as equipment and second for allowing companies to use the credit to offset 90 per cent of their total tax bill rather than only 50 per cent.

At the same time, Carter is expected to proposed a series of modest, but significant, changes in the tax code designed to help carry out his pledge to "reform" the present tax system and reduce some inequities. These provisions include:

A gradual end to the $12,2 billion tax subsidy for exporters who have set up dummy "domestic international sales corporations" - a tax break Carter critized vigorously during the election campaign. Carter was reported still undecided on whether to proposaseout of another foreign tax break he castigated during the 1976 presidential race - the ability of the corporations to defer indefinitely the taxes on the portion of their income earned abroad.

Treasury Secretary W. Micheal Blumenthal has pushed hard for retention of both breaks, to help stem the nation's trade deficit.

Edging into the traditional taxfree status of states and localities bonds, by offering a choice of continuing to issue tax-exempt bonds or of issuing taxable securities with the federal government making up the difference in their interest outlay.

Extension of last year's congressional crackdown on the abuse of tax shelters. However, the administrator's plan would not affect the biggest remaining shelter - real-estate investments.

Repeal of the 1976 provision allowing firms to deduct the cost of prepaid legal insurance, which pays employees' lawyer bills such as health insurance underwrites their medical costs.

Decisions on the size of some of the major tax reductions were expected to depend largely on how big an overall tax cut Carter figures he needs for economic reasons next year. The Presidents has said he wants the tax cut to help sustain the economy end to offset the impact of coming increases in Social Security and energy taxes.

However, while the Social Security bill is scheduled to be signed into law today, the energy bill won't be completed by Congress before mid-January - possibly too late to be considered for the budget. As a result, some aides say Carter may have to send up a "supplemental" message altering the tax-cut package somewhat.

Carter indicated yesterday he would not seriously object if the witholding rate reductions he is proposing were delayed until Oct. 1 - the start of the new fiscal year - rather than taking effect July 1, as the administration wants.

The President was warned by the chairman of the Senate Budget Committee, Edmund S. Muskie (D-Maine), that his panel would most likely balk at revamping the present congressional budget ceiling to accomodate an early changes of witholding rates. If the House felt the same, the July 1 effective date might not be possible .

The package Carter outlined yesterday was, as expected, trimmed substantially from the massive tax-revision proposal the administration was considering in September. Before that the White House had been toying with a far more comprehensive proposal, but withdrew in the face of opposition from business and congress.

Proposal scrapped along the way would have ended the special tax treatment of capital gains - profits from the sale of Stocks or other assets, eliminate the double taxation of corporate dividends, reduced the maximum tax on investment income to 50 per cent from the present 40 per cent, and ended the depletion allowances for minerals other than gas and oil.

Carter had pledged to propose all of these as key elements in "reforming" the tax code. An end to special tax treatment of capital gains has been a long time dream of liberal tax "reformers."

The White House has hinted it may proposed some version of these and other "reforms" in a second package, to be submitted in 1979 or 1980. However, plans for this still are uncertain, and most tax experts seem agreed Congress would be unlikely to pass them in later years even if the President proposed them.

The proposal to eliminate the telephone and air fare excise taxes is the one new element in the President's latest package. Most of the others had been disclosed previously, many as early as mid-November.

It is not immediately clear the proposals would affect various income groups. Tax experts said much of the impact depends upon which option Carter picks in the case where advisers have provided choises on tax-cut size or composition.

In general, the administration appears to be aiming for the average tax cut for the individuals of $340 to $350 for a family of four, in cases where the taxpayers earne$100,000 a year or less. The cut would be lower for higher-income taxpayers. For families earning more than $200,000, the proposals would mean a small tax increase.