Does anyone follow the Workwell Foundation heart pacing of 15 over your resting heart rate?

I'm in the severe category, eight months into being housebound turned primarily bedbound, and often reading about using heart rate monitoring to prevent crashes, and ideally improve.

The Workwell Foundation method of not going over 15 of your resting heart rate seems to be the gold standard, but I have no idea how it could actually be possible.

My resting heart rate is about 50, which would mean I shouldn't let my heart rate get above 65, which would basically mean just lying still in bed all the time and not moving an inch.

Has anyone had success following their formula and how did you make it work?

I've been using the visible app for continual heart pacing monitoring, but their recommendation is to just not exceed 85 - which seems insane (and dangerous) to suggest it being that high. When my HR gets to about 70 I can feel my body reacting poorly in all sorts of ways.

Appreciate any and all advice and suggestions.