@grwoolf makes a really good point, and I’d like to re-emphasize it with the following real world example with #s. I’ll also try to answer your original question now that I understand it:
Last evening I did a ride that lasted 2hrs 45mins. My average power was 124W. As my FTP is 240, using the average power approach you are suggesting, this would be a recovery ride (52%) with a TSS of about 50-60. Correct?
Here’s the real situation: It was a weekly 40 person hill interval suffer fest consisting of 19 Vo2Max hill efforts. My NP for 2:45 including 15 mins warmup and 15mins cool down was 0.89 for an NP of 213 and TSS of 216.
Which approach, AP or NP, do you think better describes the effort?
To answer your question regarding calculating Average Power before the ride:
All of my TR interval workouts that I do indoors I put on spreadsheets. I exclude warmup and cool downs. But I have listed the interval duration, interval intensity and same for rest intervals (intensity and duration). If you did something similar, it would be easy to develop a calculation for the Average Power (mathematically, its a weighted average of the duration and intensity). And if you want to include warm up and/or cool down to the calculations, that would be easy enough to add. As inferred by @grwoolf and my comments, its not clear what usefulness it would be to you, but you could do it.
btw: The reason I put these workouts on a spreadsheet is that I use it for tracking Time in Zone (TiZ) progressive intervals in order to improve fatigue resistance . . . a topic for another discussion.
btw: the place that I have found Average Power IS useful is on a single interval. If, for example, you are doing a 20min hill climb that has a relatively constant grade, you can compare (after the fact) your NP to AP to get get your Variability Index for the climb. It’s a good indicator of how steady a climber you are. When I am really on target I have hit as low as 1.002. For comparison, last night’s overall ride had a VI of 1.74.
One final suggestion: If you are looking for other TR user feedback on Average Power, you may want to consider editing the wording in your top level post to be clear about that.