Help: Fender sizing

[Bugged]

I have been looking to order some Kerscher flared rear fenders for my superbeetle 1303. I am trying to figure out how much wider I will need to accomodate my wheels. Any input is appreciated, and if you have flared fenders if you could post your wheel and fender size.

Wheels:
15x8" 3.5" backspacing, I believe this equates to 15x8" ET -25.
Tires:
225/50/15

According to Kerschers website, the 4cm widened outside accommodate up to 9" ET-39. According to this, these 4cm widened fenders should fit with room to spare.

I am a bit confused though because my rear tires poked beyond stock fenders quite a bit. Here are some pictures for reference.