You are underfinned if when you reach top speed and "push" laterally across the top of the fin it lets
go and spins out.
My personal "rule of thumb" for fin length is the fin has to be greater than 1/2 the board width or OFO
The wider the board and the greater the footstrap off set, the more the 1/2 board width applies.
On a Futura or other freeride board with inboard and outboard footstrap positions, you can generally
run a smaller fin (perhaps < 1/2 the OFO width) with the footstraps in the closer to centered positions.
The smaller fin (or underfinned) also depends alot on sail size and overall board speed.
I have really tiny (<25 cm) speed fins that are great if you can head off wind and get them up to > 25 knots board speed.
At 30 knots + board speed they are solid as a rock, but if you have conditions where you can't get them up to speed, you can push them loose any time you like if your speed is < what it takes for these tiny fins to "hook up" and stay hooked up.
Hope this helps,