After 5 years of v.Bowling in anything and everything, here is my best advice for fin placement and sizing on most v.bowls at this time. I have a few variations on the v.Bowls fin - the 8.5" and the 9", both in thick or thin foil, this blurb covers those four options. This is not a list of commandments, but a reference for you from my experience and talking with others. Suitable substitutions in a pinch are also the Greenough 4a, and Skip Frye 8.5" flex fin from true ames.
For lighter to medium weight surfers or shorter, quick waves (beach break, weird reefs, or most any windswell waves) use the 8.5” thin foil v.bowls fin placed in the center or towards the back 1/3 of the box (for boards up to 7’6). for boards over 7’6 go to the 9”. for boards 7’2 and under add the FCS bonzer side bites to the thin foil 8.5” if you have the option for more release off the top and more response from quicker pumps.
use the thick foil versions for heavier surfers or more powerful carving in good waves. Place the 9” in the center to slightly forward of center in the box.
Use the 9’ thick (heavy footers) or thin (most surfers) foil in all v.bowls over 7’10.
For reference, I’m 6’2 and 195 lbs - I ride the thick foil 8.5” plus fcs bonzer side bites in my 7’2; thick foil 8.5” in my 7’4 (single fin), and thin foil 9” in my 8’ (single fin).
Two great rules of thumb for fin placement to start: place the trailing edge of the fin, in inches, the distance that the fin is tall; so for a 9” fin, go 9” off the tail. This is a good rough starting point.
If the board feels too loose or like its letting go from the bottom turn too soon, move the fin back. if the board feels like it doesn’t release when you lay off the bottom turn, or like its tracking/has a mind of its own or a small anchor under it, move the fin forwards. Move it in increments of 1/4” until it feels like a rocket ship thats reading your mind.