+Jason Packard suggested charging a 5 point UB per college the wizard knows, up to a maximum of 30 points. It's a decent suggestion, but not the one I'm going with, because there's another problem I want to solve.
Wizards Can Cast Too Many Spells Each DayA well-designed GURPS Wizard has Recover Energy-20, and a decently designed one has Recover Energy-15. Even assuming the wizard gets no other energy reduction on spell cost and performs no other tricks, he can cast 12-30 FP worth of magical spells every hour. That's not a huge problem if most of those spells have a very short duration, but if some or all of them have durations of a few days or more, the effects can be over-the-top. It's trivially easy for a decently prepared wizard to equip everyone in the gaming group with magically glowing rocks while fortifying their campsite with deep trenches, long wood berms, and freaky magical mists. This can be a bit upsetting in play and highly upsetting in world design.
A large part of the problem is that wizards can convert the otherwise mostly unused Fatigue attribute into problem-solving. Sure, non-wizards can use Fatigue for extra-effort, helpful in combat or sometimes to break open a wedged door or something, but it doesn't compare to a wizard being able to spend Fatigue to fly or shape earth. Fatigue as a resource for casting spells needs to go.
Fortunately, there's already a system for that: Threshold-Limited Magery.
Threshold-Limited Mages Can't Cast Enough Spells Per DayThe suggested limits for Threshold Mana are a Threshold of 30 and a Recovery Rate of 8, "a power level roughly equivalent to that under the standard rules". But that drastically underestimates how much magic a standard wizard can cast per day (trivially, a 10 FP spell every hour, not an 8 FP spell every day). The Recovery Rate needs to increase to around 30-50.
Dabblers, Extra Threshold, and CRBM MagesMy proposal is that wizards (and other "arcane" spell-casters such as Elementalists, Necromancers, and Demonologists) should get a Threshold of 5 and a Recovery Rate of 5 when they buy the 5 point Magery advantage. They can by additional levels of Extra Magical Ability at 10 points/level, with each level increasing Threshold by 10 and Recovery Rate by 15.
This system means that dabblers in magic - the hypothetical bright thief with IQ13 and a few Easy spells - can only cast a few small spells per day safely, and needs a few days to recover after any significant use of magic. Which can only be expected, as dabblers aren't mighty wizards. A professional wizard would have Extra Magical Ability 2 or 3, giving a Threshold of 25-35 and a Recovery Rate of 35-50. Professional wizards can cast many more spells than dabblers, but they still face a pretty hard limit. If they cast more than about 75 energy in a day they're guaranteed nasty effects on the Calamity table, and recovering that Threshold requires 1-2 days without casting any magic at all.
Of course, rewriting the Calamity table to make it easier to go over threshold without having horrible things happen permanently is useful, too.
Sample Artillery MageHere's the same artillery mage above, slightly modified to use Threshold Limited Magery with these rules. With a Threshold of 25 and a Recovery Rate of 35, he's more powerful in a tactical situation than the original, but not as powerful when there's lots of time available since he can't just keep casting small spells. His skill levels are also slightly lower, since he had to drop a level of Talent to pay for the Extra Magical Ability.
Damage 1d-2/1d; BL 20 lb; HP 10 ; Will 15 ; Per 12 [-15]; FP 14 .
Basic Speed 6.00 ; Basic Move 6 ; Dodge 9.
DisadvantagesClueless [-10]; Oblivious [-5]; Obsession (To become the world's most powerful fire wizard; 12 or less) [-10]; Pyromania (12 or less) [-5]; Sense of Duty (Adventuring companions) [-5].
Quirks: _Unused Quirk 1; _Unused Quirk 2; _Unused Quirk 3; _Unused Quirk 4; _Unused Quirk 5. [-5]