They should know, but then again, so do I. Problem is, you need to find a shop that has this tool: http://www.parktool.com/product/p/bo...ket-facing-set

It's not cheap, and as I said, my LBS doesn't have it because the need for it doesn't justify the cost for them.

I think the eyeballing method is good enough, because it should work as long as it's close to spec--Campy specifies that it can be .8mm + or -, I think. And worst case, you can use some shims like the ones sold here:
http://roguemechanic.bigcartel.com/