In mid-September, I promised I would discuss the new rules for minimum IRA withdrawals for those who reached age 70 1/2 in either 1985 or 1986, so here we go.

You will recall that the mandatory withdrawal date -- normally April 1 of the year following the year of reaching 70 1/2 -- has changed a couple of times and has finally settled on Dec. 31, 1987, for both groups.

If you reached 70 1/2 sometime in 1985, you now have to take your minimum distributions for 1985, 1986 and 1987 by Dec. 31; if your key year was 1986, then both 1986 and 1987 withdrawals are required by the end of this year. But things revert to normal for those who attained 70 1/2 in 1987; that is, the initial withdrawal must be made by April 1, 1988.

There are three different methods for calculating the required withdrawal, and you may select whichever method you prefer. If you are interested in conserving as much of your IRA principal as possible, you will, of course, select the method that results in the lowest minimum; but you retain the option of withdrawing more than the minimum, up to the total in the account if you wish.

The amount distributed is taxable as ordinary income in the year withdrawn, but there is no penalty for anyone over age 59 1/2.

Under the "life expectancy method," you start with the total balance of all your IRAs on Dec. 31, 1986, to which you add the amount of any voluntary withdrawals taken in 1985 or 1986 (for the 1985 class) or in 1986 only (for the 1986 class). Then divide this total by your life expectancy (or the joint life expectancy for you and a designated beneficiary, if applicable).

This life expectancy is based on your attained age on your birthday in 1987, and not on age 70. If you reached 70 1/2 in 1985, the next step is to multiply the answer to that division by 2.8; the multiplier is 1.9 if you were 70 1/2 in 1986. Then subtract from that amount the total of voluntary distributions already taken -- the same amount that you added in the first step above. The result is the total amount you must withdraw by Dec. 31 to cover your 1985, 1986 and 1987 mandatory distributions for the older group, and the 1986 and 1987 withdrawals for those who reached 70 1/2 in 1986.

The second option -- the "percentage method" -- is a little simpler. The first two steps are the same: Start with the Dec. 31 value of all your IRAs, to which you add voluntary withdrawals made in 1985 and 1986, or in 1986 only for the younger group. Then multiply this total by 15 percent if you reached 70 1/2 in 1985, or by 10 percent if you were 70 1/2 in 1986. Subtract any voluntary withdrawals already taken and added in the first step.

The answer is the amount you must withdraw by Dec. 31 to meet the minimum distribution requirements.

The third alternative is the trickiest of the three. It's called the "regular method" and requires that you go back and compute the minimum mandatory distribution for each of the three years on a year-by-year basis. You must determine the IRA values on Dec. 31, 1984, 1985 and 1986 (or just 1985 and 1986 for the 1986 class), then divide each annual value by the applicable life expectancy for each year.

You should then subtract from the minimum from each year any voluntary withdrawals made during that year, to determine how much more -- if any -- you should have withdrawn. Then add the amounts arrived at for each year together to determine the total that must be distributed by Dec. 31, 1987, to meet the minimum requirements of the law.

I must warn you that the explanation of the "regular method" in the IRS instructions isn't entirely clear -- at least to me -- and there are differing interpretations, particularly concerning the application of voluntary distributions taken during the previous years. You should consult your tax attorney or accountant, particularly if a large sum of money is involved.

For smaller amounts, try the first two alternatives -- the IRS calls them the "transition" methods; if you're interested in withdrawing the smallest amount possible, select the one that gives you the better answer. In any case, rather than relying on the necessarily abbreviated information here, get a copy of new IRS Publication 930, "1987 Required Distributions from IRAs." It may be obtained free at most IRS offices, or call 1-800-424-FORM and ask that a copy be mailed to you.Abramson is a family financial counselor and tax adviser. Questions of general interest on tax matters, insurance, investments, estate planning and other aspects of family finances will be answered in this column. Advice cannot be given on an individual basis. Address all questions to E.M. Abramson, The Washington Post, Business News, 1150 15th St. NW, Washington, D.C. 20071.