i'd base it on size of the pot. smaller pot, smaller percentage needed for good drainage.
because coco doesn't retain as much water as soil, you'll want to maximize that portion... it already has excellent drainage propoerties.
10-20% should do you upto 5 gallon.. maybe 20-30% in something like a 7-10gallon?
it floats, so have a couple inches of pure coco to put on top, if you don't like it turning all white up top.. also helps avoid algea on perlite that does rise to top.