( Nice find in that article - I'm sure 60 to 70 % will be fine )
HI Ian - Its easy to calculate with plop - you just allow a single ring and put in loads of points say 36 to let it optimize the radius - I have some vague memory it was more like the 60% zone or something fro my mirror. Haven't used Plop for years so probably cant help you there. You need to make sure the `allow refocus ' button is checked somewhere.
No just used normal velcro but it was high quality stuff off a bulk roll but maybe not . I know that if I need to remove the rear triangle its mounted on it takes sustained pressure with a wedge for about a minute before it will start to separate ! The plate you velcro to wants to be nice and flat of course and varnish help the velcro pads to stick well .
From memory the 2.4 meter ATT telescope at Siding spring rests on collimating bolts but most of the weight is taken by just two ring shaped rubber tubes filled with air ( or mercury ) and probably pressure controlled by valves that are driven by counterweights that sense the attitude of the mirror and adjust the relative pressures in the tubes - a kind of astatic support.
This mirror is 1:16 ratio but this mirror would be I think 25 X more flexible than a 20" of the same ratio. Neverthless - amazing to think such a large mirror basically just rests on two tube annulus of air or mercury and no separate flotation points.
You could try an air ring support for fun - pump the tube up just enough that the 3 collimation bolts were able to do their job and most of the mirror floating on the air ring
