For what it's worth, I would mark the exterior roof line with tape. But just shy of the finish attachment points. I would use a razor saw and cut through both lines leaving a 1/16" uncut at the drip rail. This allows you to do most of the work without breaking anything. Then gently make the last four cuts and sand the edges until the T top part fits perfectly. I wouldn't trust the mold line to be accurate. And as mentioned take your time, be patient and don't drink any caffeine before starting.