Google engineering papers regarding their findings with open vs closed chambers quench vs no quench, it works no questions asked, it's not black magic. Not to many years ago cylinder head porting was held in similar regard, like it was some sort of voodoo. Look at how small direct injection chambers have gotten, how much compression is being run, (LT4 chevy runs 11.4 to 1 on pump 87 without detonating), with all the OE's setting quench between .035 and .045. The Quench effect is just starting at .06, and gets better up until pistons hit the head, which could be looked at as too much quench lol.
It works by squeezing out the gases very rapidly, this rapid movement of the mixture does 2 things. One it remixes the air and fuel charge, and two, the movement around the chamber limits exposure time of fuel to hot spots in the chamber. Thus reducing detonation issues. Big open chambers need more advanced ignition timing to completely burn, this is bad because the heat has more time to build creating more hot spots in the chamber, the mixture is also more static and the fuel starts falling out of suspension, it sits on the hot spots, as well as creating more load on the crankshaft, pistons, and rods.
There are caveats to the "rule of .040" thou, like large bearing clearances, aluminium rods, that would cause a need for larger static quench, but your effective or running quench is actually smaller because of metal growth and collapse of the oil film tolerance. Some engine builders have successfully run motors with as little as .025, on N/A motors with tight tolerances and high grade parts. So how the builder prefers to build the motors oiling system, thin oil vs thick oil, HV oil pump, Standard volume oil pump, ceramic coatings, coated bearings, etc., will also affect the final tolerances, and ultimately the final static quench.
In your shoes, I would ask him to shoot for at least .045, unless he has a specific reason not too like one I listed above. In the end its your motor, your money make it the best you can with what you have, for me that means proper quench.
