Expect your plant to double in height after you flip. So get the max height you want your plant ( A - B ) and make sure your plant's current height does not exceed half that amount. (A-B) / 2 = Max plant height before flipping.
I would recommend going a bit lower then that just to make sure you don't run into height issues.
( A - B ) >= C * 2
A : Distance From soil to light
B : Light distanced recommended for flower by manufacture.
C: Height of plant.
Most people flip after two months of veg when they grow photoperiod plants. I like to give them an extra month or two, depending on the size of each plant and the size of the room I'm growing in.
Good luck finishing up your grow!