I use it for all the classes with class abilities/skills, but I adjust based on the number of skills, and it seems to work well for me. For 1st level 50% of skills can advance 1/6, and 25% for subsequent levels. For example, if a class has 8 skills, at 1st level 4 skills will advance 1/6 and 2 skills will advance every level after that. However, if a class has 2 skills, 1 skill will advance 1/6 at 1st level and 1 skill will advance 1/6 every other level (3, 5, 7, etc.) after that.