Mortons Spoiler Site - Spellcrafting

Total mana you can use in any spell:
[ (Magic Skill x 3) + (Spellcraft Skill x 7) ] ÷ 24 = maximum mana

For example, with Conjuring 40 and Spellcraft 30
[ (40 x 3) + (30 x 7) ] ÷ 24 = 13.75 mana

Spells require 1/10th their skill level in mana to cast normally

For example, Skin of Stone requires Conjuring skill 20 to cast
(20 ÷ 10) = 2 mana to cast normally

Spells can be empowered by using additional mana, in multiples:
1x, 2x, 4x, 7x, 11x, 16x, 22x, 29x

For example, Skin of Stone can be cast at
1x2, 2x2, 4x2, 7x2, 11x2, 16x2, 22x2, 29x2
(that is: 2, 4, 8, 14, 22, 32, 44 or 58 mana)

Higher multiples (also called, "steps") yield better bonus(es) or damage, typically not longer duration or greater range
-- Grusk points out that you get more duration from Aura