Optimal Magic Levels
Your magic recharge time depends on what percent of your magic charge level is left: refilling from 0% to 25% is considerably slower than refilling from 75% to 100%.
The "optimal max magic amount" is the amount that gives you the fastest recharge time when you cast that spell from a full charge. If your max magic is lower, it takes longer to recharge because the low percentage means you're regaining points slower; if your max is higher, it takes longer because you need to re-gain more points.