10% perlite is what I often do. According to Jorge Cervantes Grower's Bible, no more than 30% perlite.
I mix it myself, I also rinse and soak the Coco in cal-mag before mixing in perlite. I don't have an exact calculation of mix, I use a one gall of perlite to 5 gal pot of Coco. Different mix for different set ups, I use that mix because it works best for me. The more perlite the faster the Coco dries. I like mine so I can water/feed every two days by flower.
I'd definitely use perlite, one it helps create opening for oxygen to get to the roots and it also holds some of the nutrients.
What percentage do you use vs coco?
Do you mix it yourself or get it premixed?