@HAL-1999: I'm all for keeping things metric, but in the case of 1/4", 3/8" mount standards you've got to think of those as universally recognized names, not imperial units. The 1/4" 20 UNC mount standard uses a 0.635cm wide hole, yes, but that does not mean we can simply call it the 0.635cm UNC mount standard.. Replace <1/4"> and <3/8"> with <smallhole> and <bighole> in this thread and you'll find we're sticking with metric for all our measurements.

Regarding interactivity and "live" communication with the audience during a show, given the usual 30-60 second delay:

My measurements below, all relating to the 1/4-20 UNC mount standard: (bear in mind that I didn't have my caliper at hand, so the measurements may be off by ~0.5mm)