I found this write-up from Scott J. to be very helpful: https://uphillathlete.com/forums/topic/ctltrainingpeaks-and-tfna/
For hard muscular workout that has a very high local muscular endurance factor with a (disappointingly) low HR such as an uphill ME workout then I pick a number that reflects the recovery time before the athlete feels ready for another such workout. An ME workout for a high level athlete like David Goettler or Luke Nelson will take them days to recovery from and based on the actual workout I have assigned I will give these a TSS of 150-200.