Not established, but Joe Friel (of The Cyclist’s Training Bible fame - amognst others) has mentioned the following:
An endurance athlete would do something similar—but with a new wrinkle to get TSS. After determining the tonnage for the session you’d have to decide how hard that was relative to what you normally do. If it was an average strength workout for you it could be given a TSS of 50. As the tonnage increases or decreases from the norm with each session you would assign a new TSS number either higher or lower than 50.
For example, you may find that your average weightlifting session is 3.5 tons. That would yield a TSS of 50 and would mean 14.2 TSS per ton (50 / 3.5 = 14.2). So a harder 4-ton session would be a TSS of 64. An easier 3-ton workout would have a TSS of 36.