"As reference on any vehicle I have that doesn't have a coolant overflow tank I do a simple calculation to provide the appropriate air compensation to prevent overflow dV(volume change)= V(volume of coolant system) B(volumetric temperature expansion coefficient)((T1(final temp)-T0(initial temp). So for this application dv= .016088M^3*.00057(104C-21C)=.0007m^3 or 26 ounces. So in this case with engine off I filled the system complete to the top of the rad then opened the petcock at the bottom of the rad and pulled off a little less than a quart, made a mark on the rad to note the proper level for the system once burped, started the engine and this is when I noticed the issue with the fluctuating."

...or...fill up the radiator, let engine run with cap off. Once thermostat opens add coolant until level is just below filler neck. Monitor temperature as engine continues to run with cap off to burp system, also monitoring coolant level. Shouldn't take more than about 20 minutes. Replace cap.

Your engine is operating at a perfect temp. Why overthink it?