You could make some blanket statement that "amounts are estimates" or "amounts prior to 2000 are estimates" somewhere or a cover letter or some such.
Seriously, if you can't figure it out, who can? There is no way I could figure out my "lifetime earnings" even with all my tax returns for my life (does the IRS even keep them for my whole life?).
So they know it's an estimate- that's not a lie, you are doing it to the best of your ability (that said, don't waste a lot of time with it). Really, it is just a piece of paper they are going to file away, it will never be reported to anyone.
Even when I do my taxes, where the numbers are supposed to be exact, I usually can only estimate what the previous year's medical expenses were, I don't keep spreadsheets on my entire life. You do the best you can.
Yeah, a stupid hassle, sux.