Does Normalized CP60 = Functional Threshold?

Saturday I rode for four hours- the first and last hour were by myself, the middle two hours were with Gewilli and Chris Burke. On the surface, the ride does not indicate anything impressive. Average speed of 16.8 mph and average watts of 171. It is easy to achieve these numbers on flat roads, by myself. But the Variability Index (VI) is 1.28 which indicates a lot of climbing. The TSS for the entire ride is 300 which is the highest I have done in one day, YTD. The data which matters the most to me though, is the small inset showing the CP60 (peak 60 minutes of average watts). The average is 191 watts, but the normalized average is 250, which is the current FT setting I'm using is WKO Peaks software. We were not going at race pace or turning ourselves inside out completely (for the most part), but we were not exactly slacking either.
FT means different things to different people. I use the number as a target during my longer intervals (those of 15-20 minute duration). Some workouts require me to go "FT+10%" and knowing this number comes in handy. But as a trusted source points out, my heart rate indicates that the FT I am currently using indoors may be too high. Some of this is because of the fatigue of a long day at work, some of it is because of overheating while indoors, or lack of a fresh supply of oxygen (I'm in a basement with no windows) and some of it is 'cardiac drift'.. I'm not completely convinced yet that I should aim lower during these workouts, but I'm thinking about it. The "VI" of a windtraner workout is typically exactly "1.0", which means that it's a constant steady grind- nothing like a ride in the rolling hills. If my Peak Normalized CP60 is achieved on a ride with lots of big hills, then translating that to a windtrainer workout needs to be given some consideration. At the end of the day, what needs to be evaluated is whether using a slightly lower [FT] target is going to force the same level of adaptation as a slightly higher one. Two back to back 15 minute FT intervals are supposed to be hard. The 2nd interval is supposed to show a little bit of weakness relative to the 1st one. I look at it this way.. training should be harder than racing. If I'm trying to condition myself to be a successful contributor to a breakaway that forms with 5 laps to go in a crit, it makes no sense to do a 15 minute interval at anything less than FT [or more]. If I'm trying to condition myself for a 10k time trial, it makes no sense to do a 15 minute interval at anything less than FT [or more]. One thing is for sure though.. using only a HRM can lead you into the weeds relative to the effort you're putting into an interval. The information is important, but by itself it can mislead.
Thanks for reading.