Choosing piston

The machinist did say he found a piston that would put it around 9.7. The guy at Cleggs suggested the 23 cc piston saying he’d run “10:1 all day long”. I think quench is closer to ideal with 10:1 as well.
Makes me a little nervous. I know I can always go to a thicker gasket.
I hadn’t thought of the altitude. Maybe 10:1 is the way to go especially for more low rpm torque.