The way I do magic is based off the spell slot system, but converted to a point system that the magic user can draw from. a 1st lvel spell slot is one mana point, 2nd is 2, 3rd is 3 etc.
So if you have 4 1st, 3 2nd and 2 3rd slots that would be 6+6+6 so 18 mana points.
But wait, you also get to add your spell casting ability stat bonus. So as a Bard for example you use charisma and you have a +2 in Charisma - you add 2 points to your mana.
So with 20 mana, you can cast spells as you see fit. 20 first levels, 10 2nd, or combo. As you cast spells your mana goes down.
You still use the same spell level reach by character level as in the rules.
This is across the board for all spell casters. Clerics/Mages/Sorcerors/Warlocks etc.