WE WERE OFF by a mere factor of 1,000 in an observation we made the other day. In case you were one of the few readers who did not catch the mistake and write us a letter about it, in a discussion of the annual gross product of California -- and whether that state's economy was the world's sixth or seventh largest -- we wrote that the gross national product of the United States was $2,088.2 trillion for a recent year.

Some of the more charitable readers wrote to suggest that a mechanical error had transposed the comma, the decimal point, or both, because what we had wanted to say was $2,088.2 billion. It would, of course, be easy to shift the blame in this manner to others. But it would be wrong. And besides, it wouldn't work.

The only thing to be said in defense of this particular miscalculation is that we are so bullish about America that we clearly overstated the case. We do know, and reluctantly admit, that the GNP of the U.S.A. is three, not 3,000, times larger than that of the U.S.S.R.

Which brings us to -- naturally -- the campaign of Gov. Reagan, who absolutely corrected stated that the annual economic production of California makes the economy of the Golden State the seventh largest in the world. Lately, Mr. Reagan has been on the receiving end of some strong criticism for a few of the figures he uses in his basic campaign speech.

He subsequently explained how he happened upon some of the controversial numbers and ever since, we have had a recurring nightmare. Mr. Reagan confessed to reading and clipping newspapers, then added: "like any other speaker, I'd see something and I'd say, 'Hey, that's great,' and I'd use it." So did we. But that's not our nightmare. It is that Gov. Reagan will pick up this number, use it, be attacked for doing so by this paper and -- well, you see the general drift. The governor was right about the size of California's economy -- and so were we. But we hope he will just take our word for it, not our numbers.