Thanks for your answer paxx,
but my main question aimed for the percentage system(100%maximum) what imo is better then the most used point sytem, where you get more and more mana points with level ups. In the percentage system spells would need less mana after useing them much, and you would gain a higher regeneration factor for higher levels. The advantage of the percentage system is imo a more realistic and controlable use of mana, the example was the shield that would f.e. use 40% of your mana with f.e. a regeneration factor of 20% you could hold it up for a lmedium time but not endless.
If you cast powerful spells which use much mana percent the shield would drop. Or the opposite, shield needs 10% you get 20% regenaration, then you could hold up the shiled until you cast to many spells.
I fear due to my bad grammar this might have been misleading before.