From this thread:
https://www.cyclechat.net/threads/heart.225112/ (30 months ago!)
The HRmax = 220 − age formula it is widely thought to have been devised in 1970 by Dr. William Haskell and Dr. Samuel Fox.
[23] It was not developed from original research, but resulted from observation based on data from approximately 11 references consisting of published research or unpublished scientific compilations.
[24] [into patients with heart issues]. It gained widespread use through being used by
Polar Electro in its heart rate monitors,
[23] which Dr. Haskell has "laughed about",
[23] as the formula "was never supposed to be an absolute guide to rule people's training."
[23] While it is the most common (and easy to remember and calculate), this particular formula is
not considered by reputable health and fitness professionals
to be a good predictor of HRmax. Despite the widespread publication of this formula,
research spanning two decades
reveals its large inherent error, Sxy = 7–11 bpm. Consequently, the estimation calculated by HRmax = 220 − age
has neither the accuracy nor the scientific merit for use in exercise physiology and related fields.
[24]
Best is (probably) Tanaka, Monahan, & Seals (2001): HRmax = 208 − (0.7 × age)
[21]
To find one's resting HR, take it in bed on waking, before sitting up or anything, staying
relaxed.
As
@YukonBoy said an RHR of around 43-46 is not "very low". Indurain was reputed to have an RHR of 28bpm and I know at least one other who's recorded this, using a Polar HRM. But 43 is well below average.
One's HR range/reserve (HRmax - RHR) is useful for coaches who can specify "80%" say, and that means HRrange x 0.8 plus RHR. So say a rider/runner's max is 195 and their RHR is 35 then an interval session - say 8 x 2 minutes at 80% with one minute between, would ask the rider/runner to accelerate till HR hits 163(ish) and hold it there to the two minute point, and then ease off before the next interval starts.
Another runner/rider might have stats of min 50 max 160 so they'd need to target 138 for the same session.