I'm in the middle of replacing all curtains on an '86
Sovereign as well! See photos attached. I'm using a measurement of 26.5 inches from top to bottom of finished curtain. And it might be too tight. I would probably do them at 27.25 in. if I started over so the curtain completely conceals the track from view.
The real
key measure is where the straps end up when stretched. I wanted a very tight look when finished. The ones I'm replacing sagged and looked awful.
After finishing the curtain hemming, sew your first set of two glides top and bottom then test them on the entire length of the track (it's not the smae measure the entire length) to see if you like the fit. I am eyeballing the placement and checking frequently. It's tedious, but I'm happy with the result. Good luck!
See
my other thread on elastic here.