I think of IQ like unforced errors. Bad fouls and giving up easy baskets on defense, poor shot selection and dumb turnovers on offense. If I can, I try to limit their exposure until they get to C+. If I have to start them due to promises, it means 0 distribution, and shuffling them in the lineup to keep them away from the opponents attack points.
How fast they move up depends a little on work ethic and HS GPA. Smart, hard-workers can get to the C+ range by the middle of the year, with 20 minutes practice time on a set. Guys lower in those categories will still be in the C- range at that point.