I have not had to put any autos I grew into 12/12, I've only grown autos on 18/6 or 16/8 with no issues flipping to flower on their own.
Ideally I'd do 18/6 but with my current setup and lights that equates to something close to 50DLI and its to much,
So i do 16/8 and it comes out to 37.5 DLI, and that's close enough to 40 for me, 40 being estimated max DLI before CO2 supplementation.
My lights are restricted at 40-60-80-100% dim settings, no in-between. If I could get them down to 20-30% I could go back to 18/6, but I don't feel like buying their 300$ controller to do that.
DLI is not a myth, its a scientific measurement that has shown to work well in most applications.
Eg my lights at 40% dim and 30" hang height at 18/6 is 37.5DLI
Then in flower my lights at 60% dim and 30" hang height at 11/13 is also DLI of 37ish.
I used a quantum par light senor to measure my tent and lights, I even made maps of it.
You can see the setup in week 2 and the results in week 3. It took me a while to do and it was a bitch and a half, but now I know with my Medic Grows mini sun 2's
growdiaries.com/diaries/191541-exotic-seed-honey-mints-grow-journal-by-m0use
Good information by numbers with regards to 12/12 and if it only flowers at 12/12 then yea maybe it is a photoperiod. I have also heard of autos taking 6-9 weeks to start blooming. At that point might as well have grown a photoperiod. Its unknown to me if switching autos to 12/12 will help them flower like in photoperiods or if its when they want. Still I don't think it will hurt them to try though. Just make sure its getting the same energy input so they can grow well. and that input is measured best in DLI.
Good Luck!