Ah Jeeze thats a tough one. My Robbins came with a line too but I quickly found out it was wrong. Your top should have a bend up where the rear bow staples to it. That header will need to be 22 1/2" (well, that was my measurement) from the chrome trim. If you attach the bottom too short/long this won't line up properly and won't be tight when its 22 1/2".
I waited and had most of the top attached before I stapled the top to that rear header. I figured if I messed that up, it was game over.
