The pyramid is a good basis.

The way you've described using it is best for when all you do is learn the spell, but you don't gain ranks/level/skill for that particular spell. You then use some stat elsewhere for the character to determine how good you are with it. This is basically how D&D does it, with the other stat being the character's level.

If you allow the character to gain ability for a spell, then it would be better to take the pyramid and turn it onto its edge. This works by requiring that you to have a lower level spell with its rank greater than a the rank of a spell that you have at the next higher level.

For example, if you have a first level spell with rank 4, then you could have a second level spell with, at most , rank 3.

Note that each spell can only be used once for a spell a the next higher level.

Continuing the previous example, and assuming the character only had the listed two spells, before you could gain another second level spell at rank 1, you would first have to have another first level spell with at least a rank of 2.

To look at this another way, from the 'top-down':
If you want your character to have 2 third level spells at rank 2, each, you will need at least:
* 2 second level spells at rank 3, each
* 2 first level spells at rank 4, each

Having your magical mechanics work this way forces the characters to be very good at the basic spells, while not so good with the more advanced, powerful spells.