I converted a van to a camper one time. Measured for the side window, cut it perfectly... went to install the window and found I had cut a rectangualr hole and the window had rounded corners... each corner had a clear view from inside to outside.....
I cut a frame from 1/2" ply, stained it and mounted it on the outside between body and window... looked pretty good... and saved my butt from a lot of rain on trips....
If I had the problem that originated this thread, I would have done the same thing, made a little frame and gone with it, unless the piece that had to be recut was small.
Often, I'll slip the tape 10 inches so I start on a '0'....