Jim,
Initial setup; 1/8" - 3/16" freeplay at the fork, then tighten jamb nut. It should stay adjusted after that. If it doesn't there flexing somewhere. My guess is it's where the cable mounts to the firewall. If that's not solid, your firewall is flexing in as you depress the clutch instead of disengaging the clutch. Every time you push the clutch pedal down, you're putting about a 200# load on the firewall cable mount.
Second place I'd look is if your pedal set is twisting when you depress the clutch.
Anyway, here's how I set mine up. Got the BBK clutch cable & quadrant kit for late Mustang, and BBK firewall adjuster. The quadrant I grafted onto the upper clutch pedal. Had to cut about 1/3 of it off, but there's still plenty to work with. Looking at where the cable comes through the firewall, I made an aluminum puck about 3" dia, with a flat rubber gasket between the puck and firewall. The puck distributes the load over a wider area so the firewall won't flex, and the gasket cushons any high spots between the puck and the fiberglass so it won't crack. The puck bolts to the firewall, and the BBK firewall adapter fits inside the puck. No flexing. All clutch pedal movement is transferred to clutch fork. Haven't touched it since installation. When clutch needs adjustment, turn the firewall adjuster out. No crawling under the car. e-mail is
jwalsh21@ix.netcom.com if you want shots of this.