Well, first things first.
The South to North postal service seems to be in better health than the North to South one. Perhaps the horses are losing shoes coming over Shap or there might be a Highwayman or two still active in the Crowden area as I still haven't received my original or replacement FRA 2022 calendar.
With regards to the Forerunner 35 and HR info mine tells me that my average resting is 49, not bad for an old codger. However it sometimes tells me that my HR has been as high as 210 whilst running even though I don't feel as if I'm on the limit.
How accurate they are I do not know but I suspect that variables like how tight is the strap, how sweaty you are, and in winter, do you wear it over a baselayer sleeve cuff will come into play.
I don't tend to look at the HR when I run as I'll frighten myself but no doubt if you do then you will get a feel for it and hopefully be able to get some benefit.
I think if you want really accurate HR info then you best get one of those ladies bra type things that professional footballers wear.