Value help

While there's no one mathematical formula, I'd say that for recordkeeping using 65% as an estimate of the drop from 4 to 3 would work for me. But I would hasten to add that the Red Book is the least reliable "value" estimating tool since the "values" of the 2012 book were assembled in early 2011. I always spring for a coin dealer newsletter/supplement (greysheet) just to get an idea of the real coin market--keeping in mind that dealers are only going to pay the listed amounts to each other and not a collector wandering in off the street.