calibrating a soldering iron
IIRC, the calibration method by Reloadron refers to the station, the iron and tip is simpler.
Once a station is calibrated its needn't be adjusted further and the scale printed on analog station are matched and permanent. The values are military driven standards, that is 1oF variation for the station, and no more than 10oF variation at the tip under no iron load test conditions, such as touching a conductive trace or terminal.
Irons can be adjusted periodically. Hakko 936 manual recommends checking tip temperature whenever tips are removed such as for replacement or changing tip type; it can also be quality check for performance for worn tips.
All these activities are not mandatory for hobbyists, but if you wish to use as low heat as possible to reduce the risk of damaging small parts such as all SMTs, the procedure is fairly simple and can save you future grief.
Hakko provides a simple method for their irons using the tip thermometer and to summarize, the tip must have liquid solder.
What is measured is the heat of the solder on the tip, not simply the naked tip, if not it can be many degrees lower.
The tip thermometers are mV DVM scaled to the thermocouple, but you can do this yourself with a good DMM, the right size bead K-type thermocouple [ the bead must be small enough that the entire bead or its 2 sense wires are fully immersed in solder] and a good feel.
Because the original test rig is a simple design, copies, counterfeit or used models are easily found on eBay for $10-20 and includes a bag of thermocouples, they work. The key to the tester are the Hakko thermocouples, they retail for ~ $20/bag of 10, $5 for the eBay copies. Counterfeit Hakko FG-100s sell for about $40 on eBay.
I think using solder would be an even less reliable method. As you say, heat is a very dynamic quantity to measure; it's always flowing. To melt solder, you have to overcome losses too, and that's why we set our irons to 350°C when solder melts @ 250°C.
Hakko sells an uninsulated
iron thermometer ($220). I suspect that it is designed and calibrated to compensate for losses. Hm, I wonder what tool they used to calibrate the thermometer? And what they used to calibrate that? And ....
Anyway, the thermocouple has a very tiny thermal mass. Close enough for me