We measured a bunch of times but were off a little bit. We wanted to use the angle grinder with the 0.040-inch metal cutting blade from the start but were afraid that all the extra heat would bubble the paint. We loaded the grinder with a grinding disc to clean the entire perimeter of the hole, getting rid of any burrs so the top frame would fit just right. We ended up with some bubbling of paint, which the exterior flange covered, but we cleaned and touched it up with the Rust-Oleum anyway. We think that had we cut the entire hole with the grinder, it would have been much worse.