[DragonX]

% chance to learn = (base chance + (2 * level))/1.5

The level being multiplyed by 2 is your overall level..Here is the base chance table..
1 int - 0 %
2 int - 0 %
3 int - 1 %
4 int - 2 %
5 int - 4 %
6 int - 8 %
7 int - 12 %
8 int - 16 %
9 int - 25 %
10 int - 36 %
11 int - 45 %
12 - 55 %
13 - 65 %
14 - 70 %
15 - 75 %
16 - 80 %
17 - 85 %
18 - 90 %
19 - 95 %
20+ - 100 %

Also, mental level and luck affect whether you learn a spell or not...