RE: Cutting down 70 cm fin to 55 or 62 cm
I think you may be overlooking something here.
The differences between a 70 cm Formula race fin and a 62 cm race fin (not formula) are more than just the fin span.
The foils on the shorter fins (not a cut off 70 cm) will be thinner and the chord will be shorter.
They test these things before they make a production run and get all the proportions balanced out.
Also, what sail sizes are you using on the X-186.
If you are on a 5.5 m2 on your Equipe and then moving that same sail to the 186....?
I hope not, because the X-186 is not going to work well with a 5.5 m2 rig... probably ever.
What you are missing in this equation is that formula boards are designed to be sailed with formula sized sails.
20 knots of wind on a formula board is usually a 9.8-11.0 m2 rig.
BUT, the big formula rigs are very flat foils, with tons of twist at the top and are designed to be sailed extremely overpowered in only 2 directions UPWIND as high as you can go and Downwind, as low as you can go. Same applies to Formula boards. Upwind and downwind is what they do. Reaching is not!
If you aren't using at least a 7.5 m2 formula type rig on the X-186, then things are going to be pretty slow and very unbalanced.
You would probably be alot more happy on a 100 liter Carve with a 5.5 m2 rig in 20 knots.
Hope this helps,