"Yes, there is some good information floating around. However, it all
boils down to, in your words, having an accurate standard available. I
do not."


This is what ham radio is all about. We make do without adequate test
equipment - we just need to put a little more effort into figuring how to
use what we have.

Take a dummy load. Use a dvm to measure the dc resistance. That should
also be the rf impedance. You can determine power going into the load by
measuring the rf voltage across it. Some of the surplus voltmeters will
measure rf voltage. Or, borrow one. Knowing the rf voltage across 50 ohms,
you can calculate power. P= e squared / R.

It is likely that the linearity of the Bird is ok, even if the power reading
is incorrect. So, once you check the accuracy at one point, you have it for
all points.

Colin K7FM