I think the spells should be less dependant on shards to work, and more dependant on stuff like path of the mage and spell mastery.
Currently mages usually get so high level that succeeding with a spell becomes trivial.
I do not like the percentage though, its either not going to scale with shards, or be extremely shard dependant (Like Now).
Luckily these are the trivial details that can be easily modded.
Sincerely
~ Kongdej