Address the spell of magic missile being overpowered for such a low-cost spell by scaling its number of dice on skill level rather than experience level. One solution is to give it 2d6 per skill level, though this means that starting Wizards can cast it for 4d6 damage, which is rather high. Another option is to make it scale on powers of 2: 1d6 at Unskilled, 2d6 at Basic, 4d6 at Skilled and 8d6 at Expert, which has the same respectable maximum as the linear system.

Note that a 4d6 magic missile at Basic for 10 Pw may not be considered too powerful for an early game Wizard (it’s statistically slightly better damage-wise than force bolt but also has the benefits of being a ray), but this is contingent on the existing Pw regeneration formula, which keeps Wizards from casting spells much in the early game, remaining as is.