For 40mm you want a guard around 53-55mm
Just an update on the SKS guards I fitted to the Voodoo Marasa.
The bike is advertised as having mudguard mounts, but for starters, the front mech position prevents any mudguard being fitted to the seat tube mudguard mount.
Also, the disc brake mounting screws at the rear conflict with the mudguard stays and need about 8mm of spacers to take them clear.
I've got the front of the rear mudguard strapped to the seat tube with a cable tie.
SKS mudguards only come with assembly fixings but nothing to fix the guards to the bike (I knew this already having read the reviews).
The only way to trim the stays, is to use a vice and angle grinder. Nothing else will touch them.
However, all that aside, once they are set up they are very good mudguards, smart to look at and they do the job brilliantly.
I'm very happy with my buy.