I had an athlete in the race today, and he is racing tomorrow. He swam great, out in the top 10, about 17 seconds off the lead. He followed the plan, and dropped out at the start of the run. Tomorrow he races the Olympic distance non-drafting, and he is ready to do well tomorrow. How do I know this? Besides using my own intuition as a coach and my own eyes which show me, his data shows me as well.
Here's his Performance Management Chart for the bike and the run, which has calculated his training stress, (TSS), from each session we've done over the months.
There are some important numbers which help show how much rest is enough, and how much is too much. CTL is the blue line, basically represents his fitness. His loss of "fitness" during the taper was only, but he dropped so much fatigue that the short term training stress, (ATL = pink line), is now less than his CTL value. This difference is positive 1.7. These values help show he has had an excellent taper, and is ready to go for the race tomorrow.
Is this perfect? No. Will these numbers mean he is going to perform to his absolute best and win the race? No, but they do increase the odds and probability that he will race to his potential at this point in time. There are a number of things that go into racing well, like mental prep, technical skill, and race execution. But so much of performance comes from training, and putting your odds in your favor that you're doing that right should help your confidence as an athlete. We will see tomorrow how he does.
Of course, if you're not using data, then you're just flat out guessing. We are all guessing and making judgments on what the right amount is, especially coaches. At least this guess as a coach is backed with data and evidence, in addition to my intuition, sight and experience.