There are 800,000 present and former postal workers out there who are asking the same question: How much do I get, and when do I get it?

"It" is their share of a $400 million settlement that the U.S. Postal Service reached last week with unions and employes who charged the mail-moving corporation had shortchanged hundreds of thousands of workers between May, 1974 and May, 1978. The money denied them included payments they had been due for overtime, shift differentials and for time spent in travel or study on behalf of the USPS.

Postal officials say that they have a lot of work to do before they figure out who is eligible for the settlement, how much each of the estimated 800,000 people is due, and whether recipients still are on the payroll or will have to be tracked down before they can be paid.

Unofficial estimates are that the maximum payment to an individual under the settlement will be around $700, with most people getting less than that.

The settlement, brought about because the USPS did not comply with the Fair Labor Standards Act in computing pay, will be based on time actually spent on the job during the period when the alleged violations took place. The time period is from May 1, 1974 to May 3 ,1978.

In most cases, employes who were on the payroll during that time period (except casuals, rural carriers, some postmasters and high-grade employes) are entitled to $6.30 per biweekly pay period if they were clerks, maintenance workers or working in some other postal craft, or $7.35 per pay period for letter carrriers.

That doesn't sound like a lot of money until you spread it among 800,000 people, for a four-year period.

Postal officials say the settlement will not trigger a rise in stamp prices, nor cost the public a penny. They explain that they set the money aside several years ago, just in case.

If you worked for USPS during the period covered by the settlement and still work there, your office will tell you what to expect. People who have quit or retired from the postal service will have the settlement checks sent to their last known address.

Postal officials expect a run on change-of-address cards in the next few weeks from former employes suddenly anxious to hear from their former employer.