So let's set an example:
For my Galvantula, I have the ability Compound Eyes, with Thunder.
So regarding Compound Eyes accuracy boost and Thunder's 70 base accuracy; will Thunder's accuracy boosted with Conpound Eyes have:
• 70 + 30 (100)
Compound Eyes adds 30% accuracy
~ OR ~
• 70 + 21 (91)
Compound Eyes adds 30% of base accuracy