New 408 build finally started

I'm shooting for 9.5 to 10:1 compression. Machinist wanted the block there before ordering parts.
I get that the deck cleanup affects which piston to choose.
Just wondering, without the rotating assembly to see how far in the hole the pistons are, how does he even measure to make a piston decision?
By compression height based on deck height..then chamber volume piston head volume..head gasket volume...then ica...then consider head flow.