Sterling says there should be no more than a 1/32 inch gap between the walls and the receptor. But whatever gap there is seems to be determined by the contact between the receptor flange and the pin thingy on the back wall. They butt, are coplaner. The woman on the help line suggested that the pin gizmo is supposed to slide behind the receptor flange, but I don't believe this. By lots of wiggling I was able to reduce the gap to < 1/32 (supposedly 2 credit cards thickness), but this was actually accomplished by grinding down the contact a bit between back wall and receptor flange.
Why is there a gap at all? Shouldn't the parts be absolutely tight against each other? Sterling says to caulk only the front 4 inches of the wall to receptor seam. Is there any harm in caulking all of it?