So, I need help from minds greater than mine!

I want to replace the rear mudguard on the Savage, with probably a cutdown trailer one, but working out which size is doing my head in.
The one currently on (well, at the moment off) it is 18cm wide, 43cm long, and the curve is 8cm from the bench, with the bottom of the curve 5cm off it. (See pics)
The wheel it fits over is a 15" wheel with a 140/90-15 tyre.
There must be some sort is mathematical way of working this out, but buggered if I can do it!!
Any help, as always, is gratefully received.

Rear mudguard help

Rear mudguard help

Rear mudguard help