It is a 20% boost to the move's accuracy. So in Inferno's case, 20% of 50 is 10; so add that to the original 50 and Inferno has 60 accuracy.
So your first calculation is correct.
Inferno is still horribly inaccurate (I thought 70 was bad on Focus Blast), so I'd suggest using a more accurate move like Flamethrower on your Ninetales.