Well my mate who is doing the work has a fully set up machine shop, with two Mills at the moment. He works with beetle engines building trikes. Restores classic alfa romeos professionally from making the body panels by hand upwards. So I am confident he can do the work.You want a vw machine shop with the proper flycutting tools really.
I trust him to do the work right. He knows what to do.If he's got to ask you then find someone else who knows what they're doing. Isn't he the one that should be finding out what to do?
It actually needs to be concentric and equidistant & perpendicular from the crank centreline, which due to case manufacturing intolerances, may not be the same as the case split line. - Assuming you want the most accurate job and don't want the pistons running crooked.Obviously the bore needs to be concentric and equidistant & perpendicular from the case split.