There is no formula. Look at it like this: if you get in a room with 99 other 44-year-old men, are you all going to have the same max. and resting HRs? Of course not. So the same forumla can't be reasonably extrapolated for all 100 of those individuals.
To find your resting heart rate, measure it before you get out of bed for three consecutive mornings then take the mean average.
For max. HR, there are various approaches. Bear in mind that, outside of getting a lab test, it's hard to get an accurate number but here's a very quick way of getting a rough figure:
https://www.runnersworld.com/training/a20806124/how-to-find-your-max-heart-rate/
Some suggest adding 5bpm to the number you get.
Basically you can get an idea of max. HR by running an interval session (or, ideally, hill repeats), with each repeat being progressively harder until you are at absolute maximum effort. This should not be done frequently as it can be dangerous to push yourself that hard.
Then enter both your max. and resting HRs into your watch. When out running thereafter, use the HR zones on there, aiming to spend 80% of your miles in the aerobic zones.