I’m glad you asked this. I prefer to use Joe Friel’s method from the Triathlete’s Training Bible but my HR threshold in TP was loaded as 165 which I know is not accurate. I’ll have to do another race to get my HR threshold but if I estimate it at 175, which I think is probably pretty close, then an average HR of 156 puts me at the very top of zone 2. Does TP determine TSS based on the HR zones you have loaded? This would explain the reason for the drastic difference in TSS.