Changing physical damage calculations?

The idea is grand: I custom-made a new earth-elemental spell, called Megathrust. I could have the weapon in question cast Magnitude8 75% of the time the effect is activated, and 25% of the time, the stronger Megathrust. All I would have to do is replace the spells in the code?

There is a large block of unused space starting at 6469 (6669 for me, since I'm using a headered ROM), and I believe its the only amount of space in C2 bank that would be sufficient for the above code. I believe I've figured out how to re-adjust pointers (if I started the new code at C2:6669, then the Soul Sabre effect's new pointer should be 69 64, replacing 6E 3F, correct?). I think I'm a lot greener at this than I thought, so I guess the big question I have must seem ridiculously basic: what should the two "??" bytes you listed be?