QUOTE - RUS_12 ---- "sit.B If you use very small part of extension in mast(and too long extension), a backlash of extension in mast cause critically wrong shape of the sail."
Huh !?!? That is crap. Why guess? .... Just follow the manufacturers recommendations, then it is easy.
If you need flexibility & you are on a budget, then you could use North gear to make things cheaper with their Carbon Xtenders. The recommended mast in numerous sails, is actually using the Carbon Xtender often with 30+cm of extension. I use mine as per their ideal mast recommendations and they rig perfectly ... and I also am complying with their specifications in case there is any warranty issues.