I would think that even a charge of 100 grains is a bit stiff for that model. The recommended charge for a round ball for that cannon from the Traditions website is a minimum of 45 grains to a max of 80 grains of 1 or 2 ffg. I'd try something in the middle like 60 or 65 grains and adjust up or down as needed. You are going to damage or split the wood in your carriage if you keep getting that kind of recoil.
BTW, the block of wood used for elevation is called the "quoin".
Cheers