Nice presentation, danw2. May I add that I have seen nominal 250 Ohm resistors so far of spec as to cause problems.
May I suggest that measuring the voltage drop across a KNOWN resistance is a good way to infer the current in the loop. That is usually adequate for trouble shooting. However if you need very accurate current measurment, the accuracy will be limited by the tolerance of the resistance. Once you have checked the actual resistance of a particular device, the accuracy of your measurement method will improve to near the accuracy of the meter used to check the resistance.
In the real world a loop may contain more than two devices.
eg: A transmitter, an indicator, an alarm relay, an I

transducer, a controller. If the voltage measurements across devices times the input resistance of the devices agree, then, for trouble shooting purposes, you may often assume that all the input resistances are close to spec. If the current through one device is off by a noticeable percentage, you probably have an off spec resistor.
I hope that this helps with your testing.
Bill
--------------------
"Why not the best?"
Jimmy Carter