when you flip to 12/12. that's day 1 of flower after first long dark cycle.
Autoflowers are a guess based on using what we can see. If you want to compare apples to apples you need to guess from the first sight of preflowers. Probably in flower 5-10 days prior depending on your eyesight. this would be comparable to a photoperiod and should be of similar length, because autoflowers not "faster" growers. they just turn to flower phase on their own.