High interest rates and the new tax law should stimulate more attention to detail when it's time to make quarterly estimated tax payments. The stakes are now high: Pay too much, and you may wind up borrowing money at 19% interest from the bank when you could have kept it for free; pay too little, and you'll be hit with onerous penalties.

The Internal Revenue Service struck ir rich with one provision of last year's Economic Recovery Tax Act. That provision called for a new interest rate on tax underpayments each year, to be set at 100% of the prime rate in September of the previous year. In September of 1981, interest rates were near their peak, so the new interest rate was set at 20% for 1982 -- up from 12% last year. The 20% rate went into effect February 1.Its application to estimated tax underpayments will be especially painful because estimated tax underpayments are considered penalties, and thus can't be deducted from next year's return as a business expense.

"The high rate should be a real deterrent to underpayments," says Fred Anton, tax manager in the New York City office of Coopers & Lybrand. "In the past few years, when the rate was 12% in 1980 and 1981, or 6% in 1978 or 1979, some companies looked at underpayments of their estimated tax as a relatively cheap source of funds."

Since casually underestimating your taxes will no longer be a prudent strategy, careful estimating and close attention to the rules will now be vital for maximizing your company's cash flow. In principle, companies are supposed to pay one-fourth of their total tax bill in each of four estimated tax installments: corporations in the 4th, 6th, 9th, and 12th months of their fiscal years; proprietors and partners in the 4th, 6th, 9th, and 13th months. But a small business -- i.e., one whose taxable income hasn't exceeded $1 million in any of the last three years -- won't be charged a penalty if:

* It pays in equal quarterly installments an amount equal to at least 80% of the taxes it winds up owing; or

* it pays, in equal installments, the amount it paid last year; or

* it bases its payments on actual earnings in the year to date, paying 80% of the taxes it would owe for the current quarter if it continued throughout the year as profitable as it has been so far.

Suppose you're experiencing a bad year. If you paid, say, $200,000 in taxes last year, but you'll owe just $100,000 this year, you can conserve valuable cash and still avoid a penalty by estimating your profits carefully and paying just $80,000 in quarterly estimated taxes during the year -- 80% of your actual liability.

Companies that start years unprofitably can benefit by basing payments on actual earnings to date. For your first estimate, due in the fourth month, you can simply multiply your first three months' profits by four to produce your estimate of profit for the year. Then pay 20% of the taxes that would be due on those profits. It doesn't matter if you manufacture Christmas-tree lights and you know your real profits will be far higher than the estimate this calculation produces: The IRS will accept an estimate based on annualization of your results so far.

For the second, third, and fourth quarterly payments, corporations can choose the period on which to base their annualization: They can annualize either on the basis of earnings up to the end of the last full quarter, or on the basis of earnings up to the end of the most recent full month. Many corporations make both calculations and then use whichever calculation produces the lower liability. (Partners and proprietors, however, must base their payments on earnings through the end of the most recent full month.)

All this means that the key to paying just enough tax is to track your profits carefully throughout the year. "You don't have to do a thorough audit for each tax period," says Michael Redemske, tax partner in the San Francisco office of Price Waterhouse, "but you do have to be able to close your books at the end of the appropriate month."

It's therefore important to work out a system for tax estimation with your accountant. "You may have a LIFO (last-in, first-out inventory accounting) adjustment every year, at the end of the year. From quarter to quarter, though, you'll just have FIFO (first-in, first-out) numbers. Your accountant may be able to help you devise quarterly LIFO adjustments," Redemske says.

Accurate month-by-month records also enable you to get a quick refund if your business falters and the records indicate you've overpaid your estimated tax. "You can get a refund usually within 45 days," says Dan Morrison, tax partner in the Oklahoma City office of Arthur Andersen & Co.

If you fall behind in estimated tax payments, however, you can't catch up and escape a penalty. Penalties are assessed quarter by quarter and, says Morrison, "The IRS is very efficient about assessing this type of penalty."

The IRS requires an extra form from firms that fail to pay at least 20% of their total tax liability or 25% of their previous year's tax liability per quarter in estimated taxes. They must show how they produced the profit estimates they used. If the IRS finds their calculations deficient, it will charge a penalty.

Estimated tax rules get even tougher for a corporation if its profits ever exceed $1 million a year. If a company has had at least $1 million in taxable income in any of the last three years, it's considered a "large" corporation. Large corporations, unlike those the IRS classifies as small, can't merely match last year's tax payment and conclude that they've met their estimated tax liability. For fiscal years beginning in 1982, they must make estimated tax payments equal to 65% of actual tax liability even if that means paying more than last year's bill. And the Economic Recovery Tax Act raises that minimum to 75% for years beginning in 1983 and 80% for years beginning in 1984.

On the other hand, if your business is organized as a partnership or a proprietorship, you have two opportunities to reduce the tax burden that corporations don't have:

First, you may estimate your income as equal to the amount you earned last year, but calculate your liability based on this year's lower rates.

Second, you can calculate your estimated tax using a method corporations can't use at all: Figure your taxes, including self-employment tax, as if you were figuring taxes at the end of the year. Then pay 90% of the taxes you would owe if you earned no other income after the last day of the month preceding the one in which you are making your payment. This method may enable proprietorships and partnerships to calculate estimated tax payments on the basis of lower tax brackets than the one they are likely to wind up belonging in.

Although this method is a good deal more complex than simply paying 25% of your previous year's liability each quarter, it's well worth considering in these days of tight money.