As I don't own a workshop manual...yet.

What is the minimum thickness for a 228mm clutch plate..

cheers Steve

I have a pocket manual for the Type 4 Kombi engine with me, but my serious VW manual is at home.

The pocket manual does not specify a mininum clutch plate thickness. It says only that the plate should be replaced:

1. if there is wear on the splined hub;
2. Runout of the disk is more than 0.5mm when checked with the plate mounted on a mandrel between centres. Measurement made at a diameter of about 195mm;
3. If loose rivets or cracked centre plate;
4. Worn, cracked, burnt or oily clutch facings.

It also recommends that a completely new plate be fitted if necessary; relining of old plates is not recommended. If relining is attempted, it must be done with special VW rivets and special tool VW783.

Perhaps someone else has the factory manual to hand...