If you care about cruise speed, you need to actually measure cruise speed.
As Julian already said, doing so is the reliable way to determine cruise
speed.

I have a stupid n00b question to add to this thread. Mooney #1
was tested on a cool, dry day in Colorado at a density altitude of
8500' using the GPS square course method. Mooney #2 was tested
on a hot, humid day in Texas at a density altitude of 8500' using
the same method and power settings. Can the results of these two
tests be used to accurately compare speed down to a single knot?

I'm guessing the answer is "yes". Whatever the answer, though, it
seems that measuring cruise speed would still be the best way to
measure cruise speed.