Your HRM won’t help you train for the trans savoie per se, you need to work out how to train for that kind of ride and then use the HRM to ensure you’re doing the right workouts.
I use mine for intervals (although as noted above, short intervals don’t work that well as it takes time for the HR monitor to catch up and then slow down again, then again short intervals usually means max effort anyway and if you’re doing them right you won’t be able to read the HRM at that point anyway through the tears and blackouts 😉 )
But for doing long steady rides / identifying the level you can sustain for a long period – as above do a threshold test (hard as you can for 30 mins, take the average of the last 20 – because you’ll start off too hard in all likelihood) and then calculate your zones off that.