I too thought this chain tension stuff on long bars was too subjective...and I don't even have a 'long' bar. Trying to balance a derailment (too much slack)...with bar wear/heat (too much tension) was too hard to do repeatedly. I use a little baggage scale we had already. The objective was to find a mass/deflection number that was just loose enough to stay in the bar nice...yet not derail if overheated (finishing the slab rather than stop and sharpen). That in my opinion was the two extremes...offer some small amount of slackness..with no chance of derailment even if overheated to a reasonable amount.
I set chain tension, on a 660 with 36" bar, cold... pulling the chain from the cutter with 1kg of force...very close to my subjective tension (3-5mm deflection). Once hot (usually after my first cut) it might be close but I reset using only 500grams of force for the following real cut. That seems to be good for me. 500g moves the chain in the center of the bar 3-5mm away from the bar. So far, for each pass (I haven't milled in a few months now), I hook the baggage scale and see if I'm still 'about' 3-5mm off the bar. I'll do this until I calibrate my fingers and do a better job subjectively. Free spinning up the chain doesn't have it bouncing around causing you guys stress. Heated chain stays in the bar grooves for more than one slab.
This BTW is the same method you use to adjust the track tension on a snowmobile (hang known mass to obtain known deflection on warm track). Another belt type driven system that reacts wildly to heat variations..doesn't like to derail/nor be too tight. I just guessed a mass and deflection for the chain/bar.
Maybe we can all have a go using a standard procedure and see if we cannot quantify this issue for various bar lengths...or teach me how to properly set chain tension if you feel mine is wrong.
For me...36" bar, hot chain, 500g = 3-5mm deflection measured near center of bar. Or for you guys down in the states....1 lb. = 0.1" - 0.2" of deflection.
