The UEM rep is correct for the KB243's.
I have the same pistons in my 340, and spoke to UEM 15 years ago on the same topic. The top of the piston is .018" over the deck. But that's to the top of the dome! The dome itself is .030" tall, measured from the flat part or shoulder of the piston (above the ring lands). That puts the shoulder of the piston .012" below the deck. If the piston were perfectly flat, that's where it would be. Since it's not flat, the top of the dome is .018" over the deck.
I had contacted them about the compression ratio, because I was getting conflicting numbers. This was their response (15 years ago!)
"the protrusion has to be treated like it is a dome so the piston is down in the hole .012 and it has a .030 thousands high dome of 2.08cc's.This all equates to positive 7.3cc's.Use this to calculate your compression ratio"