Hey @CPT_A ,
There a re many factors that can affect HR during a workout that affect HR drift: temperature, hydration status, fatigue, stress, caffeine, sleep quality, duration of workout/intervals, etc. My initial recommendation is to go with power but use your best judgement if feeling the need to reduce intensity. Also when was the last time you tested either HM or FF?
Make sure you have a really good fan when training indoors to reduce the amount of HR drift.
Here is an article and podcast to check out if you haven’t done so already:
Glad you figured out the %LTHR issue on your outdoor rides.
Cheers,
Spencer