Tables below are for XLs 1 through 8. Normally players gain 1 MP for every point of Spellcasting, plus half again as much for every point of Spellcasting that's less than their XL, up to XL8. These more effective values are highlighted. Dg and Re are the only species which have different breakpoints because their MP modifiers are multiplicative. For everybody else, breakpoints are the same and the MP bonus/malus is added after skill calculations.
Spellcasting and Invocations are completely interchangeable for the purposes of MP gain, with Invocations being half as effective. You can use this table for Invocations breakpoints by multiplying every skill value by 2 and rounding up.