[Jim c]

The roll centre adjuster was fiddley but I think it was worth it.
I made the stool out of on piece then made the appropriate brackets to suit.
I installed another crush tube in the frame to give me 2 mounting bolts with the ability to move up or down.
To stop the load laying the stool over I linked the L/H to the R/H via the bar.
The bar has an adjuster to change the rod length as it goes up the rod goes out.
The adjuster links (rose joint) also takes some of the sideways load on the mount but also making for accurate equal adjustment from left to right.
Finding an equal starting point is easy for the roll centre as both stools have a datum hole in the back, it is a matter of just screwing a bolt through them and the frame.
Then do equal turns of the adjuster to set roll centre.