Probably not. What I learned from my first "actual" FTP test is that first you need to get rid of your anaerobic capacity to test your lactate threshold. What I did (back in november of last year) was ride a 5 minute incremental interval that started at zone 1 and ended going as hard as I Could for the last minute until I could no longer hold it. then, after 5 minute rest I would go as hard as possible for 20 minutes. The result was 272 watts for those 20 min. From that I took 95% and that was my FTP. 263w I think.

If simply taking 20 min power was useful then my FTP back in June would have been around 325w. Since my steady 20 min power for a 40 min crit was about that.