dude this is the one thing i dont like about ASM skimmers, no ref at all to sump water level.
i have the ASM G3 and just got the ASM G4xx. the G3 is in 14" of water and works nicely but think would do better in 10"ish of water, i forget the elbow hieght going into the skimmer. the G4xx has the sedra 1500gph pump unlike the G4x which has the sedra 900gph pump. also the teh G4x and G4xx are the same dimensions but the elbows going into the skimmer body are not the same though. the G4xx hole top is 11 1/4" from the bottom the G4x 9 1/2". but i have seen the G4xx labled as G4x more than once. so which one do you have? take look at this thread
troubleshooting probs with a sedra needle wheel
i do plan on putting the G4xx in 13" of water. hope this helps