When you sell shares, the proportional calculation applies to every lot and every reinvested dividend that is disposed of. shares are sold First In, First Out.
Since you said you have careful records over all time, I don't see why you should have any problem.
the proportion factor is not going to change in the future.
Future reinvested dividends or shares purchased will not be affected.
If you plan to never sell, this is all moot, unless Congress eliminates the step up basis for heirs, which is something they want to do.
