I'd assume that 1/8-1/4 of the way is WAY more than 1/4 of an inch, that said, with the air being open that far, I'm surprised at the build-up you are seeing if those temps are consistent.
Try the dollar bill test in conjunction w/ the flat edge as BB mentioned. It'd say it'd be normal to have a 1/2 dollar sized area in the bottom corners consistently, but I agree that this darkened area is far larger than I'd expect also.
pen
Try the dollar bill test in conjunction w/ the flat edge as BB mentioned. It'd say it'd be normal to have a 1/2 dollar sized area in the bottom corners consistently, but I agree that this darkened area is far larger than I'd expect also.
pen