I use to do the 20min test, but ran into issues here in finding suitable roads to perform the test. There's only one suitable road that allows a constant 20min effort and it's pretty busy. I've since switched to the CTS 8min field test. You warm-up then do 2x8min all out efforts with 10min recovery between. I then average between the two and take 90% of that to set my FTP. All my workouts are then based on the CTS power levels.

To be honest I prefer the 8min test. I find it much easier to perform indoors as well as outdoors. The number I get for FTP seems to correlate with numbers I get from workouts and races.

I tend to test more in the winter since during the summer there's race efforts to give an idea where my FTP stands. It might not be the best way to do it, but it is what it is...