Measuring central angle of a curve given arc and chord length (approximate)

[wooja978]

Hello math experts, thanks for giving me the opportunity to post here.

My maths education is limited to a 16 yr old's so your help is much appreciated.

In my attachment, I have measured A, B and C in 112 different subjects. Using these measurements, I hope to come up with an approximate index that can be used to compare between subjects.

To approximate the arc length, I am going to average A and B. C will be the chord length of that curve. My question is: how can I measure the central angle of that curve given these values? It does not have to be exact, just an estimation that can be used to subjectively describe if one subject is more curved than the other.

In fact, it doesn't even have to be the central angle. Just a discrete value of some sort. Even something as simple as a ratio?

Please help! Thank you.

Jason

edit:
B is the outer trace and A the inner trace. C is the linear distance between the midpoints of the straight segments, and the straight segments are NOT included in neither A nor B.