I have the 2008 Manic 4,2 sail, and has at the base the spec : Ideal Mast 370 - 17 IMCS. To use this mast, I must use also the big 30cm extension at 25cm. I can rig this sail with my 400m mast - 19 IMCS without extension. The question is which is the Ideal mast ? Do you suggest to use the 370 mast with the 30cm extension or the 400m one without extension?
The 4.2 Manic should rig on the 370 mast as recommended on your sail. I don’t think you should have to use 30cm of extension unless the sail is very old. In any case, the sail will work better with the shorter mast. It makes the sail feel softer and more alive. You can use the 400 but the sail will be stiffer feeling and more difficult to sail in overpowered conditions.