Good question! I’ve always used 10 TSS per 1,000′ of gain, but I’ve often wondered if I should adjust it for skimo and treadmill sessions. My first thought was to use 2/3 for the up and 1/3 for the down (so zero for treadmill sessions and almost zero for skimo).
I’ll ask Scott J. to weigh in as well.
The most important thing with TSS fudge factors is to be consistent. Andrew Coggan (the creator of TSS) would probably roll his eyes, but I think TSS is more useful as an athlete-specific fatigue measurement and behavior gamification* than as an accurate indicator of fitness.
* By gamification I mean using points to motivate ourselves to favor goal-specific activities rather than non-.