This mostly works like GURPS Magic's Improvised spellcasting.  There are several changes.  The "weird verbs" like Create, Control, Transform will probably be house-ruled around.  College Skill assumed Magery was capped at three, and we want Improvisation to start being useful around 5th Circle and get more so by eighth, so the new table is as follows:

No points for each spell at 16-

One point for each spell at 17-20

One and a half points for each spell at 21-24

Two points for each spell at 25+

Add them all up to see what your college skill is.  So a character with Create Fire, Shape Fire, Ignite Fire, and Fireball all at 20 would have a college skill of 4.

And it's still capped at 20.

Note that college skill is not added to any stats, since it already incorporates your IQ and Magery modifiers.  Also, it's expected that characters getting most use out of this will have IQ 17 and Magery 6 or so, so a single point in a spell gets it at 21, and ten spells gets you an improvised skill of 15.

I'm considering removing the cap and adding a difficulty modifier of -1 per Circle of the spell.

Improvising a spell should involve a magic theory roll, I think, with some penalties based on its complexity (see the SpellDesign rules).  Optimizing (the process by which complexity is changed with an inverse change in Circle) an improvised spell should be impossible.

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Normal GURPS Improvised magic works like this:  you have a College Skill which is roughly 1/10th your total skill level with all spells in that college.  If you want to improvise a spell, and it isn't under a weird verb (Create, Control, Transform), you roll vs. the college controlling the Verb of the spell, then the college controlling the Noun (e.g., Weaken and Water to create a protection-from-water effect).  16s always fall, 17s and 18s always crit-fail.  It takes twice as long and two more energy points than a learned spell.

Create is done with the normal Create spells, which are already so flexible as to be effectively "Improvised".    

Control is done with a single roll vs. the Noun at -5.  15 is an automatic failure, 16-18 automatically critically fail.  Also, they cost 3 more than usual.

Transform is done with a roll vs each Noun, at a penalty related to how far apart they are.

There's also "Runic Casting" which involves learning Runes which correspond to each College, then drawing them out on paper or sand or stone and casting the "Activate Runes" spell on them.  You can carry around tiles (runestones) with the runes printed on them, and use those instead, at a penalty if they aren't enchanted.  Or you can trace runes in the air at a huge penalty.
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