You would have no problem with drainage using 50/50. If fact this may be too much drainage. Most people i know, including myself, use anywhere from 20 to 35 percent perlite. I usually start seedlings in ffof with around 33 percent and have never burned seedlings. But have heard others say different. But to me 33 percent is enough perlite to dilute the ffof where it wont burn your seedlings.