I have a Manic 4.5. The specs of the sail says that i should use a Gaastra 100 RDM. Does that mean I can not use this sail with my
Gaastra 75 RDM?  What will happen if I use a mast with less carbon?

YOU CAN CERTAINLY USE YOUR GAASTRA 75% RDM NO PROBLEM. WE DESIGN THE SAILS TO RIG ON THE BEST MAST AVAILABLE. THE GAASTRA 75% MAST HAS THE SAME BEND CHARACTERISTICS AS THE 100% RECOMMENED MAST- IT JUST DOESN'T HAVE THE QUICK RESPONSE AND ACCELERATION THAT THE GAASTRA 100% RDM HAS.