Calculating rings

[Willy Tschager]

My usual approach placing rings is trail'n'error. I wonder if there is a mathematical way or even a tool which does it for me? I have these cylo:

1 1 50 -37 -10 43.75 0 0 0 0 -43.75 0 10 0 48\2-4cylo.dat
1 1 50 -37 -10 40 0 0 0 0 -40 0 3.125 0 48\2-4cylo.dat
1 1 50 -37 -3.125 40 0 0 0 0 -40 0 3.125 0 48\2-4cylo.dat


and need the external ring.

1 1 50 -37 -3.125 4 0 0 0 0 -4 0 3.125 0 48\1-4rin10.dat

is too large cos the distance is not 4 but 3.75. So ...

w.