Hi Alex,
The power from the sail is one thing, but I also believe the power from the foil is very important! I usually use my mastbase in the front with a long fuselage (105 to 115cm) and more to the back with a short fuselage (100cm). It's a pretty sensitive tuning part! You can create a lot of stability out of using the right mastbase position, so make sure you try a lot yourself. But my setting is just behind the middle on a short fueslage, and more to the front with a longer fuse!
I'm not really changing the position much for a sailsize. as I also use different wings with a sailsize. For example a 620cm2 with 8.9 , 560 cm2 with 7.9 + 6.9 and 420cm2 with smaller, pretty much using the same position depending on the fuselage!