Well I got back into the shed today. I decided I'd work on dialing in the rear bump steer and try a few experiments and see what resulted.
I made a new spread sheet that graphs the bump steer in relation to ride height. With the wheel bolted on I was able to work out what the aproximate ride height would be and where that would fit in to the total suspension travel.
I made a few educated guesses when I welded my new shock mounts in as I didn't have a wheel I could use to check the ride height. I wanted ride height to be about in the middle of the shock travel. I have 170mm of wheel travel and ride height came in at 90mm of travel. That's close enough for me. That gives me 80mm of compression before I hit the bump stops.
I figured that when the wheel alignment is done they set your toe adjustment at ride height. So all measurements need be referenced to ride height. In the graph below I've set toe to zero at ride height (90mm) and worked the bump figures back from there.
With the tweeks I've made to the setup there is virtually no bump steer from about 20mm below ride height and up. With the wheel at full droop there is only 5mm of toe out but with a couple of mm of rear toe added at the wheel alignement so I don't think this will be a problem. I don't reckon I'll be able to tune it any better without changing the points where the arms attach to the chassis.
For comparison I've included the graph for the stock Ford arms. I'm very happy with the improvement.
Cheers