It's been a really long time since the car was painted and we added the stripes.
I believe the hood stripes determine the start point.
After those are on that then determines the top of fender stripes, which then determines the side stripes.
When we did it we first taped all the stripes to the car starting with the hood using the layout provided by the "Mustang Boss 302" book of Farr's.
If you use his diagram it appears the fender top stripes are straight. But that's just a line drawing using a hood that is flat across the back, which it is not.
We can't post copyrighted stuff here, but this image has been copied so many times, it's probably in the public domain now. This is but one of hundreds I found in a quick search.