In my HX experience, clad and backers are ordered as MIN or NOM. If cladding is not to be machined, as in a fixed tubesheet single pass exchanger, or pass plate welded to the tubesheet it usually is ordered as Nom thickness. If gasket surfaces are to be machined in the cladding it is usually ordered as Min thickness.
Same idea can apply to the substrate. The clad plate suppliers will know what allowances to apply, if any are required. So for example for 3/8" Min cladding the supplier usually would apply 1/2" plate.
Consulting with the supplier insures you order and get what you need.
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand