Choosing piston

From another thread, I asked how the machine shop decides on a piston. We've determined I need somewhere in the 23 to 25 cc dish to arrive at 9.5 to 10:1 compression on a 408 stroker. He wants pistons there before boring to 0.30 over.

Got this reply and I understand it except how is deck height after milling determined without pistons? I've seen technical ways but do they actually do it?

By compression height based on deck height..then chamber volume piston head volume..head gasket volume...then ica...then consider head flow.