RE: When to use the wave fin?
Agree with Ola above : the Drake wave fin is great for freeriding with a NP NR 5.4 sail in 20-25 knots wind (outboard and back x 4 straps; 65kg). It's a bit small with a NR 6.2 (a few spinout) and way too small with a Jet 6.9 (lots of spinout).
The 31 cm freeride fin is way too big with 5.4 sail in 20-25 knots for my light weight : board tailwalks. It's fine with both 6.2 (15-20 knots) & 6.9 (12-18 knots wind) sails, however.