Unless I'm reading that wrong, at, say, 10th level, I've got 13 spellcraft ranks, and a modifier of +7 from Int. Total mana of twenty. But I want to cast two 5th level spells, that's ten mana gone. So I can cast 4 5th level spells a day is all?
I plan to implement some form of 'recharge' every hour spent not casting spells, but can't decide on the amount to give back.
Perhaps as much Mana as your highest spell level.
Another option would be to have a ritual or incantation (UA) that refreshes a portion of Mana by performing an extended skill check against level-scaling DC. Failure means a bad effect occurs, such as damage or exhaustion.
However, you have read that correctly. It's not many spells at once.
Perhaps the Mana equation needs to be reworked, something like Mana equal to (Spellcraft + stat) x2.
Conversely the Mana cost itself could be halved.
Either way, I want to not repeat the psionics dilemma wherein a Psion using their best powers during a level-appropriate encounter blows their wad completely. That should be avoided.
Likewise the math shouldn't be complex with bigger numbers. Mana shouldn't be like using PP, with a cost equal to the effective manifester level of a power.
Also, once I figure out how, I'll also be adding unlimited usage of lower level spells as a Versumage levels up.
For instance, once access to SL 3 is gained all SL1 effects don't cost mana.
Conversely, there could be a discount of mana as one levels up so that even though an SL 4 effect normally costs 4 Mana it would be reduced by 1 or 2 Mana because of X reduction at Y levels. That would make the low ones cost nothing to very little, while the high ones don't drain everything.
edit: Added Mana Efficiency when gaining spell levels 3, 5, and 7. It reduces Mana cost by an additional 1 each time.
edit 2: Also added 7th level spells at L19 to maintain even progression.