Errors and Mistakes

[Manly Man]

When you gain a new spell level, you reduce the max MP gained by a number equal to that spell level. By the time you hit 17th level and learn 9th-level spells, you'll have taken off a total of 45 MP compared to the norm. The MP at each level would look something like this.

1st: 0
2nd: 1
3rd: 1
4th: 4
5th: 4
6th: 9
7th: 10
8th: 17
9th: 19
10th: 28
11th: 31
12th: 42
13th: 46
14th: 59
15th: 64
16th: 79
17th: 85
18th: 102
19th: 118
20th: 135

Of course, you still get your bonus MP as normal, and can even take the Extra MP feat as a way to help soften the blow. By 20th level, if you've gotten your casting ability up to a reasonable 30, you're still getting over 200 MP in total from that alone (204, to be exact). With Extra MP, you'll get twenty more on top of that.