To be honest that 5% is just a rule of thumb I have read here or elsewhere and don't know if there is much science behind it.Originally Posted by RapDaddyo .
For me, a 5% decay for double the duration beyond 1hr is the best I can expect. Without specific training at the longer duration, I find my results closer to 10% decay.
I took a look at my own files and it seems that best my NP for 2 hours is more than 95% FTP and that is somewhat repeatable effort. I had only one 2 hour "steady state" workout from last summer which was only 90% but that was a hard workout not test or race. If I recall right you are a hard and steady specialist and probably have better reference points?
Anyway I still wouldn't rush to raise FTP to 270w after an hour and half at 240w.