OK, so I’ll kick my own old thread, as I finally got around to doing some more testing, and thought I’d provide an update. (previous posts go over how I have been doing this.)
@Dr_Alex_Harrison - I thought this might interest you.
I’ve now done 3 tests - 2 a while ago, and one just recently.
The results in mg/L of Sodium for the tests are:
800mg/L (5/2020) 92F
920mg/L (7/2020) 87F
1148mg/L (9/2022) 94F
Note that these are whole body sweat concentration calculated from the forearm sweat concentration using equation 3 from: https://www.gssiweb.org/sports-science-exchange/article/sse-161-sweat-testing-methodology-in-the-field-challenges-and-best-practices
As @PrecisionHydration mentioned, there is quite a bit of variability here, so I haven’t ended up with a ‘precise’ number to use. Also I found that my left arm always produced higher concentration, by 12, 16 and 24% respectively, so as the overall concentration went up the left arm difference went up as well. I think I have controlled for variability about as well as is practical - washing arms with RO water, waiting for onset of sweating, calibrating TDS meter with calibration solution among other things. This has grown to no longer be a ‘simple DIY’ solution - based on the results I can’t recommend anyone trying this. The main reason to do something like this is to practice running experiments, not to get useful data 
At the end of the day, my assumption going into the testing was that my sweat concentration was likely about 1g/L based on surveys, etc, and the test results are consistent with that.
I also measured a sweat rate of about 1.6 L/hour on my trainer at 94F, so during the 2 hour test ride I did I lost about 4 grams of Sodium during the test, or almost 11 grams of NaCl. Even though these numbers are approximate, I do think it is helpful to roughly quantify these to guide replenishment. If I were to get all this from some 1g NaCl tablets I have, this would have been 1 tablet every 10-15 minutes on the bike, which surprised me.