I've been tinkering around it a lot, my recommendation, find a comparable player, drop the attributes back 3-5 points or 2 points in overall and you're good. Right now, the only 3 guys I have in the upper 50's are Gatewood, Gettys and Forbes ie. very raw high upside guys. Most of the high schoolers are low 60's, college guys more low to mid 60's.
Highest ranked guy I have is Nola at 72, which projects him rated around a #4 starter range come next spring which via K-Law & Kiley McDaniel is very realistic. I have Burdi, Lindgren and Howard in the high 60's as well because I think they are quick to the majors-reliever types (Will obv. adjust Howard if hes drafted as a SP). Rodon is up there as well, only issue I have is Kolek rating too high b/c of his elite fastball.