The flare on the slip on side of the gibson is about two inches deep, so plenty of tolerance. I would cut on the conservative side and trim off more if needed. The instructions have a pic that isn't great where it shows the measurement for the cut coming from the bend but there is a lot left to interpretation. Like I said, cut conservatively and trim off more if needed. I cut once and it worked fine. Since it's hard to get both sides' measurements perfect, I made a second measurement from the corner of the factory muffler to my cut mark to make sure it was in the same spot on both sides. It's really not hard at all.