Is using RPE is likely the best bet?
Anyway. I think a 30 minute to 45 minute lifting session is a minimum of 30TSS. I bet though on my solid, hard lifting days I get a 60TSS.
Shouldnt it all be a score of how long its going to take to recover form said workout?! Like If if lift Monday, usually my legs are tired or weakened until Thursday.
If you have never tried it, do a lift then go for an SST ride, I bet your power numbers are higher at the same HR!