Buy both caps and you don't have to guess. You can just look at the hatch cap and replicate the slot.To fit a Fabia arm you need the following:
Fabia hatch cap, Fabial Estate arm and blade.
Don't go for the Estate cap unless your good at guessing exactly where the water will come out.
Never had to do this on any of the three different caps I tried. Must be different on older Leons. I've an old Leon.Touran/Polo cap will need trimming down all the way around the bottom to fit properly. About 1/2cm. Easily done with a Stanley Knife, belt sander etc