There's a couple ways to set deck height. You can mock up the four corners - or you can have the shop that square decks the block figure it out. It "should" be easy if the shop has modern equipment that actually allows suare decking. It's a simple matter of setting up the block in the machine, then plot each corner of the deck surface. I know for a fact more than a couple locally say they square deck and simply can't - they dont have the right equipment for it. "Square decking" is not just "decking".