You can add a field to your bike computer’s display that shows the current accumulated TSS on your ride. This is true on at least Garmin and Wahoo computers, and probably others as well.
Sweet spot is a pretty large range, and with the way TSS is calculated, it gets even bigger. At the low end, an hour of sweet spot would be 0.84 x 0.84 x 100, or about 70 TSS. At the top end, 0.97 x 0.97 x 100 is about 94 TSS. For your 90 minutes, that’s anywhere from 105 to 141 TSS.
If your FTP is set too low in TrainingPeaks, the same ride will end up having a higher TSS than it should have. For instance, if you rode at a 90% effort, you would get 121 TSS in 90 minutes. But if your real FTP is actually 5% higher than what TrainingPeaks is set to, then you were really only at about 85% (I’m simplifying the math here), and your TSS would be 108.
TSS has a very non-linear relationship to average power. A few sprints can make an hour workout have the same TSS as an easy 2 hour ride. I’ve got a small hill in my area that’s a 20 second all out effort. I can pick up 8 - 10 TSS points just from that. On a spirited group ride, you can end up doing some hard efforts just to keep up, and that’s not really in the spirit of the workout. I asked Frank about that here:
Levels 5 - 7 in group rides while on Sweet Spot Plan
Personally, I don’t think missing the goal TSS by +/- 10 - 15% is a big deal as long as I achieve the overall goal of the workout. In this case, a sweet spot workout, not a VO2 or Sprinting workout.
Maybe most important is how I feel the next day and whether I’m able to do the next workout on the plan. In the SS3 plan I bought, the group ride days are followed by a zone 2 day, so it’s easy, and then there’s a recovery day, so even if I overcook the group ride, I’m not expected to do an Over/Under workout next. This is perhaps the real value in the plan I have: it seems well structured so when I’m tired, I’ve got a recover day or week coming up.
So, in summary, I try to focus on the intended goal of a workout, recognizing that the final numbers might be off a bit. Fortunately, my body doesn’t know or care about these numbers. I just aim to get close enough to the right effort to get the adaptation that will result in improvement.
Having said all that, some of the workout descriptions have TSS estimates that just aren’t accurate:
TSS conflict in Sweet Spot plan