A Virginia taxpayer has discovered that his state levies its income taxes on an "unfair" base, and he wants me to expose the practice so that it can be corrected at once.

I hate to disappoint him, but as far as I know, all state income taxes are computed on the same "unfair" base and it is not likely that adverse publicity will cause any state to alter its procedure.

My irate reader states his case thus: He and his wife earn a combined total of $30,000 a year. "Federal income taxes withheld from these salaries come to $5,939, and another $1,755 is withheld for Social Security taxes. Our net income is therefore $22.306. That's how much we actually take home.

"However, when it comes time to calculate our Virginia income tax, we must begin with an income base of $30,000, which is obviously unfair. We never received $30,000. Never saw it, never had our hands on it.

"All we ever collected was $22,306, and this is the only amount on which we can be fairly taxed. The state should have the same amount for personal exemptions ($750) and the same standard deductions ($3,200) as the federal, and then tax us on what's left. The way they do it now is most unfair, and I hope you can do something about it."

Is it unfair to levy state income taxes on gross income? Should state taxes be based only on what's left after federal taxes are paid? If you made it worth my while (15 cents, cash), I would be willing to argue either side of the proposition. But it really doesn't make much difference which side is right or wrong philosophically.

From a practical standpoint, tax collection is easier if the taxpayer must begin his state computation with the same figures he reported to the feds. So that's the way the states are going to write their laws.

If my correspondent thinks he would save money by being taxed on $22,306 rather than on $30,000, he's naive. The Commonwealth of Virginia must extract X dollars from him in order to pay its bills.It can do this by charging the present tax rate on $30,000 or by charging a higher tax rate on "take-home pay" only. In either case, income times rate would equal X. If $30,000 is 34.5 percent higher than $22,306, the Commonwealth can get the same number of tax dollars out of my correspondent by increasing its tax rate by 34.5 percent -- for example, from 5 percent to 6,725 percent. Reduced income times inflated tax rate will equal precisely the same X.

The exemptions and deductions are also delusions, like the high trade-in that's offered on your used car. What counts is the bottom line, regardless of what kind of cosmetic arithmetic is used to arrive at it. And the bottom line that taxation must produce is easy to figure. It's the number of dollars the politicians spend minus the number of dollars they borrow on your signature, with your children as cosigners.

POSTSCRIPT

If you moan when you receive your income tax forms, think of how much worse it would be not to receive them.

The federal forms carry this smallprint instruction to postmasters: "This matter must be forwarded and delivered without payment of postage due." However, Charles E. Dent of Woodbridge discovered recently that his Virginia forms do not carry a similar enjoinder.

When forms go astray, a recipient who wants to get them back to the intended addressee learns that he must pay the forwarding fee. In Dent's case, his forms went to Green Bay, Wis., by mistake. Some goodhearted soul in Green Bay had to pay 35 cents to send them back to Virginia. Either that, or throw them in the trash.

Question: Shouldn't the Postal Service put as high a priority on delivering state tax forms as it does on delivering federal forms? How complicated a measure would Congress have to enact to achieve that end? Why hasn't it been done?