Library of Howling
To find out the upgrade cost of a spell (i.e. raising the spell’s maximum stat):
Desired Max / 5 / (1.75 + other limitations) = point cost of the spell (where 1.75 = the -.75 base limitation of your spell tree)
You have a spell called Fireball that costs 7 to cast for 1 point of damage and has a current max of 21 (this 3 points of damage if you max it out (this would require a power of at least 21 in the spell tree you have fireball under)).
After raising your power to 35 in Fire Magic you decide you want to raise your Fireball spell to match this new potential maximum. Your Fire Magic spell tree has a base limitation of -0.75. Your fireball has no extra limitations to it (extended casting time, etc). This means your current Fireball spell originally cost you:
21 / 5 / (1.75+0) = 2.4 points
Decimals are rounded down if .5 or lower, and rounded up if .6 or higher, resulting in a 2 point cost for your original Fireball spell.
To achieve your new desired max of 35, the new cost would be:
35 / 5 / (1.75+0) = 4 points
So 2xp must be spent to bump up your Fireball spell’s max from 21 to 35, changing it from a potential 3 damage max to a potential 5 damage max (7 points per 1 dmg, 7×3=21, 7×5=35).