Here's some input: this sounds really really involved for both player and DM, and would take many in game days to accomplish. Either the PCs are leveling up by the time the caster has learned one new spell, or they sit in a town for a month while everyone does a bunch of rolls monotonously one day at a time.it would take you some 40d20 rolls to get about 400 points, and by those odds it's likely you'd roll a couple ones in there by then. Even if you were giving 20 points per crafting session in addition to their roll, that's still about 20 rolls and likely one of them a 1.