The point of FTP is really only to establish intensity levels to be used in subsequent training, right? Therefore, I would use 225 watts as your FTP because as you start doing 2x20s or 3x20s it will become apparent whether or not you are able to sustain that wattage over multiple intervals.
I personally have defined FTP as 0.95 of my monthly 20-minute test because I can tell you that doing a fully motivated 1-hour TT every month would be REALLY hard to get up for. Until my USA Cycling license requires me to certify my FTP, it's just for my own training purposes anyway and 20 minutes hurts bad enough as it is.
--Steve