I sorry to tell you that it probably cannot be done with so little information. If we knew that each circle is uniform as far as its radius is concerned then maybe. There are eleven circles on that target.

Let's say the overall target has radius. That means the innermost circle must have radius .R

However, the picture in the link suggest that none of that is true.

If it were true, then we can find the area of the first red ring and divide by the total.