Updated: Dec 7, 2020
I've been looking for an example like this. Tracy Paul-Hardin Gravlin messaged me to let me know that her workout was pretty easy and she could have kept rolling. I opened her workout, and it did look a little too easy. Her heart rate was really steady at an average of 105 bpm. Her set max heart rate after testing is 165, so ~72% of max is around 115-120 bpm. She was under on that target, as well.
These two pieces of evidence (subjective feel and approximate heart rate target) indicate that her FTP may be set too low, and I might suggest upping it.
But first we need to make sure it isn't just a coincidence she that she felt great and her heart rate was a little low, so I went back to her last FTP test (see the screenshot below). Note that the last two minutes were about 20 watts higher than the average. This tells me she was holding back with more to give; maybe she was nervous about going too hard.
This gives us a third piece of evidence of FTP set too low, and I recommend Tracy move her FTP up 9 watts to 145 and see how she feels next week.
Remember: data does not tell you what to do; it helps you make better decisions.