Manningham has almost identical stats to Santonio Holmes - in three seasons, Holmes had 140 receptions for 2,295 yards, a 16.4 average, and 25 touchdowns, while Manningham had 137 receptions for 2,310 yards, a 16.9 average, and 27 touchdowns. I gave Holmes a 4.5* rating, but Holmes also added 810 return yards, while Manningham had none. Holmes also compared more favorably to Ohio State's all-time best receivers (Carter, Boston, Jenkins) than Manningham does to Michigan's top receiver, Braylon Edwards (252 receptions, 3,541 yards, 14.1 average, 39 touchdowns). I'd probably go with 4.0* for Manningham.
I hope that that helps. The real struggle will be with the many players who will grade out at 2.5* to 3.5*, and how to differentiate among them.