Brian,
First thing you do is find "center". Easy to do if you drop the windshield in place. Then use the two lower holes in the frame, find the center there and wal-LA!
I simply take a 2" roll of masking tape and run it front to back, in one long strip using the center mark to line things up. Then from the outside edge of this strip, at the dash area, I mark out 11"s. I then run the tape foreward to the nose, "tapering" the front down to about 8 1/2"s.
I also start to taper the rear down, from the front cowl almost immediately. The outside edge of the rear stripe ends up just about 3/4" from the trunk edge.
Hope this makes sense!
DV