Now that I've done my first real tests (outdoors), I think doing the test outdoors was a mistake. Here's why:
1. By looking at my previous power files, I was able to estimate my ftp within one (1) watt of what the test revealed. (In other words, all I gained from my test was a sense of certainty about my estimate, plus some fitness.)
2. Outdoor tests aren't repeatable. For that reason, the amount of valuable information you can gain is very limited. Trends mean little, if anything, from one outdoor test to another -- especially as temperatures, winds, clothing, etc. changes. Indoor, on a trainer, I could control time, temperature, wind direction . . . just about everything. Trends revealed on an indoor trainer mean something. The greatest value in trends shown by outdoor tests would seem to be to reveal how temperature and wind impact my power output. I don't need a test to tell me that.
If I want to know trends with any precision, I will do my ftp tests indoors from now on. I'm pretty certain I can estimate my ftp from power files good enough to avoid doing any more outdoor tests.