Whatever you do for counting your flower period, imo just be consistent for your future grows. For me, since my first grow, I pretty much veg for 30 days, then into 12/12. When I go to 12/12, becomes my first day of flower. For me, typically I veg for 30 days, and I know most of the time, my plants are ready to harvest right around day 95 of flower.
I think counting when you first hit 12/12 is the easiest.