Concentric Circle In Square for Music

Hi,

I'm a Masters student of Music and I'm currently working on a project that involves writing music for a picture of concentric circles inside a square.

I came to this forum because I'm studying music and not maths so i thought someone might be able to help me with this.

I'm trying to find a way of calculating this:

If the whole picture is 100%, how much of that 100% is the circle in the middle ? (photo: File:Archery Target 80cm.svg - Wikipedia, the free encyclopedia)

Or how much percent is the first red circle ?

I hope I've explained this well.

Thanks for your time,

Fred