Determining FTP
I've just begun training with a power meter. Up until now I've only been using an HRM along with speed, cadence etc (the basics). The power meter has been a huge influence on how I ride. The additional metrics are well received and I'm trying to take my training to a new level.
What I'm trying to do is get a better understanding of where to peg my FTP. I have not ridden an hour long test, nor do I wish to in order to get a real world FTP number. What I have done is take my best 20 minute score of 142 and used that as an indicator of where I stand. On my Garmin 530 I have set my FTP to 150. I'm just not sure if that is a good idea i.e. chasing after some metrics that are slightly out of my reach, or if I should use another way of setting my training FTP?
I guess I should add, I'm 67 and my training is usually moderate with some HITT workouts thrown in here and there. What would you suggest I do in order to dial in an accurate FTP?