That is a brilliant adaptation Bill! I'll give some thought to making a set of 'Radius accessory jaws' for myself.
Just great. I need to figure out how to cut such a precise angle...
Forget trying to measure the angle , it's not even expressible as a finite number.
Essentially you'll need a good sine bar and gauge blocks or the means to make and measure a block accurately.
(I.E. You make a slightly tapered block(something I usually find quite easy) and measure where along the taper is the required thickness, this is marked and the sine-bar set on mark.)
I have not yet worked out the sine bar setting but it can be derived from Ian's formula (I think

)
[edit] 5" sine bar setting is a nice round 3.0000" or 76.200mm
[Edit 2] just measured my unbranded 123 blocks and they are a surprisingly good 3.0000" (according to my 3" and 4" mics)