The book says you get one spell point per Sphere Casting Level, plus additional spell points equal to your casting ability modifier.
So whether you're a High, Medium or Low caster, if you have four sphere casting levels, that is worth 4 spell points.
If your casting mod is +0, then it stays at 4.
If your casting mod is +2, then that jumps from 4 to 6.
Some classes get bonus spell points, like the (Sphere archetype) Sorcerer.
You can also get bonus spell points per level, from general drawbacks.
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