I adjust my workouts based on the results of the workouts. I know what I should be doing during my sets at after the first rep or so of each workout. And probably before.
I guess you could call it a CP kind of thing. I know what I've recently done for 8min as a PR. So, it's a good bet to take 5 to 10w off of that and use that as my 3 to 5min rep power for VO2 work. VO2 being 3 to 8min.
If I'm doing 8min to 12min stuff, I'm going to look at my recent best 20min attempt. Take a few watts off off that, and go from there.
Then the hour? I never do intervals an hour long, I do sweetspot though. For that I adjust as I go as it's a pretty broad range. Once that first workout is in, I can look at it and have a good idea what to do.
So, I don't really hardly ever "test". If I'm going to test, it's going to be a real world effort on a former TT course going after a PR.