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