I just noticed this post now and saw nobody has replied yet. I will attempt to but, please bear in mind, I’m no expert and what I write shouldn’t be taken as 100% accurate.
To the best of my knowledge, FTP is based upon the maximum Wattage you can maintain for an hour. Therefore, using the twenty minute test, it’s going to be a bit lower than your average over that twenty minutes.
It shouldn’t take your warmup and cooldown into account but should be based on the actual twenty minute test. From memory I ‘think’ FTP is generally calculated at about 95% of one’s average Wattage over a twenty minute period. Say, for example, you hold 200 Watts for twenty minutes (I used that number simply because it’s easy to work out an answer). 95% of 200 is 190 so your FTP would be 190 Watts.
As I say, that’s my understanding and I may not be completely correct.
Did you upload your FTP test to anything like Training Peaks, Garmin Connect etc?
If you have, you should be able to go back to look at it and see what your highest average Wattage was over a twenty minute period during the test.
If you haven’t, you can still download it from your Zwift history and upload that file to Training Peaks etc. to look at the data.
Sorry I can’t help more but hopefully that’s a start.