As a GM, the solution here is much simpler than any formal system of skill point averages, etc.
Sorcerers shouldn't be fair.
When they are required to know a spell, and it's something they have the capacity to know, they know it.
NPC Sorcerers don't, or shouldn't, make a habit of revealing their spellbooks (metaphorically) to players.
That way the players never know what an evil wizard has up his sleeve. Maybe a recurring villain has a "favorite spell" that he's unleashed at them a time or two, but beyond that... it should be a surprise. Sorcery is supposed to be terrifying and unpredictable. Characters shouldn't know every available spell in every discipline, and it's the terror of the unknown that makes sorcerers as frightening as they are.
As for skill points... who cares?! If the sorcerer is good at alchemy (in your mind, not on paper), roll a die secretly, then... guess what? He succeeded. Who cares what you actually rolled? It's a game, but it's also a story.
Unrelated reference point: When Han shoots point blank at Greedo, he hits because he's good. There never was a chance of him "rolling a one" and screwing it up. Don't let dice or the major task of accounting get in the way of your story.
If your "terrifying sorcerer of vast reputation" needs to summon a demon in order to continue plaguing your PCs, by all means, ignore the character sheet you made for him, and let him cast the spell!