If you really wanted the utmost in accuracy testing for an AR Upper, you could build the rest into the table for the upper only, (i.e., so that the upper pins into mounts on the table, and have the FCG built into the table also, eliminating both human error and 'jiggle' between the upper and lower, though that's less of a factor, if at all.
I'm actually surprised no one has built such a rig (that I know of; maybe someone somewhere has) for testing uppers they've built.
That's some right fine shootin' there, young lady!