By top edge I assume you mean the top edge of the unit (we need a common term for the entire injection-molded plastic thingy), as it would be when mounted on the rack? And the distance you're talking about is the distance from the very front of the unit to the flange that projects out around the unit on three sides (sides and bottom), right? OK, I'm measuring mine (with calipers :-), and from the back of the flange to the front of the rim around the coloured insert, it is 3.80 cm at the top, and... 3.80 cm at the bottom. So no tapering. (And it's the same on both sides.) Which I don't happen to have... something to add to my tool wish-list! :-) I did try with the butt ends of a number of large drill bits, but I didn't get really good results. I mean, you can tell when it's way off (too small, and the object 'lifts' off the surface at the outer edges of the object, too large, and you get a gap in the middle), but there's a range in the middle where it's hard to tell - it all depends on where you decide the ends of the curved part are. Oh, one note: when I gave the gap between the coloured insert, and the sides of depression it sits in ("it's slightly smaller than the inset in the plastic housing, by about .5 mm"), that .5 mm is on each side, not total. Noel