Not sure wether I understand your question correctly or not ...

1. I've modified your sketch a little bit so I get 3 symmetrically arranged arcs. Unfortunately the most important measure is missing: The "height" from the red straight line to the "top" of the arc. I've taken 60 mm.

2. Then you are dealing with a right triangle: See attachment #1

3. Using Euklid's height theorem (and your measured values) I get:

See attachment #2