-1 for a first level spell, -2 for a second level spell, etc.
+1 caster level 1-2
+2 caster level 3-6
+3 caster level 7-8
+4 caster level 9-10
+5 caster level 11+
I may have that level chart slightly buggered up, as I'm doing it from memory. A bunch of my gaming stuff is packed at the moment. Also, note that the Delay category refers to the spell going off one Chainmail turn after the casting. In my games I'd probably rule that the spell goes off the following round, provided nobody disrupts the casting in the meantime.
Anyway, even if you are a pretty strict Vancian adherent there's still lots of uses for a chart like this. A moth eaten scroll with an incomplete spell. Some joker with read magic trying to cast a high level spell directly out of a spell book. Someone attempting to cast an Arduin spell that's rated a spell level beyond the scope of the rules you normally use. Basically, any situation where the casting is less certain than normal conditions.
For DCC-style "you must roll for every spell" shenanigans, just add a "1 or less" category that sets off a spell fumble and an "oversuccess" result on a 13+. While I don't mind making up spell fumbles and oversuccesses on the fly, a couple good rules of thumb for the latter might be in order. Something like this:
- Any spell with a variable based on caster level gains +1d6 levels.
- Spells with no such variable double range, duration, etc as appropriate.
- Magic Missile gains one bonus missile for each point over 12 rolled.
If you want caster Intelligence to figure into the roll, here's a chart for that:
Int 3: -2
Int 4-8: -1
Int 9-12: no modifier
Int 13-17: +1
Int 18: +2
These are the standard stat mods for BX D&D when applying a modifier to a d6 or 2d6 roll. See Initiative Adjustment for Dex or Adjustment to Reaction for Charisma.