+1 on the advice above, that's a reasonable approach if your total time in the TT position is say 10% or maybe 15% of your riding time then it really doesn't make much difference, the PMC isn't really that precise a tool.
But yeah the topic of lower power in different situations whether it's indoor training, altitude, TT position, off road, extremely steep climbs, heat, humidity, etc. has been beaten to death but there really isn't a consensus outcome to those discussions. Some feel RPE is king and workouts that feel very hard, even at lower AP/NP should be given more TSS 'credit', others feel that a watt is a watt and if you're not sustaining more power and thereby processing more ATP/ADP, O2, glycogen, etc. then you didn't really work your body as hard regardless of how hard it felt at the time. Both sides have valid points and you pretty much have to decide how you view things, how you want to track your workload for fitness/fatigue reasons and how you want to interpret the data you enter.
In the end it doesn't really matter as long as you understand how you're using the PMC tool and what you expect it to tell you. For instance, if your near term CTL ramp rate seems very low but you've been doing much of that training in alternate situations that typically lead to lower power and session TSS for normal efforts and workout durations then you might interpret that as a higher effective ramp rate for the purposes of planning an upcoming taper or insight into when you might back off on training.
Personally I don't maintain multiple FTPs or override my TSS values to bump them up for things like being in the TT position or riding the indoor trainer or at times when I train at altitude or in unusually hot or humid conditions. If I add up all those power mitigating workouts it's probably a lot more than 20% of my annual workload so some would argue I should but I prefer to track what I did for better or worse and then just make notes on my journal page about the circumstances surrounding a session and interpret things like my weekly TSS or PMC with some perspective of what led to those values.
Lot's of discussion on the topic, some strong opinions on both sides of the debate, no universal agreement on how to handle all situations so figure out what works for you and use the tools in whatever ways helps you understand your training history and helps you manage your workload.
-Dave
