i put a tape measure on the front but the fish was slightly away from the front pane and at a slight angle but in the pic you can see it's 3 inches SL, i reckon you can put another half inch onto that for the angle and distance away from the front.

i was going to clamp it against the glass but she was very skittish, so i left her.
mick
ps can't wait to get the 5 inch job. haha