Nice photos Paul..
In a perfect world, it seems the center of the target would be the same height as the center of the barrel when the gun is in it's rest, all in level... but, since the benches are not all the same height, one would like to have some adjustment on placing of the target on the target frame... if one were to be making ones own target frame, one might consider making it large enough to have some adjustment for height.. also allowing for placing the target either vertically or horizontally.
Wacky Wayne