Check out the Table on estimating TSS from RPE here: https://www.trainingpeaks.com/learn/articles/estimating-training-stress-score-tss/
Probably your best bet. Whatever you do, be consistent.
I suspect like many of us, I use Training Peaks to get Training Stress Score (TSS) and track general fitness. I find it works great for cardio sessions where I can understand the TSShr variations from both duration and intensity. However I find hat it does not work well where cardio is not being stressed much, or only intermittently: eg indoor climbing wall sessions, or weight training/muscular endurance sessions. Sometimes I feel considerably more fatigued from these than I do from a couple of hours of AeT cardio, but the TSShr does not reflect this. I know you can manually change TSS, but anyone got any experience of how best to gauge this?
Any advice gratefully received.