I do 13 off, 11 on.
I get less stressy plants and positive finish's.
12/12 is a guideline. Not the end all do all rule of bloom! The difference in yield, would be very,very hard to actually measure.
To many other variables to account for any actual minor difference.
Having tried some of the rather short bloom time experiments....You don't really see much yield change till your below 14 off, 10 on. It drops off at a bell curve rate after that. Acceptable down to about 9.5 on. The idea being your shortening the time of the actual bloom, to harvest faster.... NOT WORTH IT is what those of us trying it, found here.
Even with some of my pure Sativa's....Bodhi's Dream Lotus seems to love it!
DAMN fine strain too!
In running L-O-N-G blooming Sativa's, you can start at 13 on - 11 off to initiate. After about 4 weeks drop that to 12/12 and go 4 more. Now drop 30 min a week till your at 10 on - 14 off and slowly drop 15 min a week to get a more positive, real "finish". This helps those Sativa's that don't seem to really finish. Helps hold Some naturally foxtailing strains to tail less, sometimes to not at all...
This does take knowing your strain and when 'done' is actually "done".