It is quite a fine operation, to ensure no jagged bits intrude and there is no overlap into the tap area. The TP diameter also seems to depend upon whether or not you use a leather washer, in which case the tp would be rather larger that with a synthetic washer. The links previously mentioned cast quite an amount of light on the optimum size for the tp, especially for the Mk3. I suppose, if you make a narrower tp, then you can then always widen it whereas the reverse is not so simple.
It will be very interesting to know how you get on.
A