Setup Polar HRM for more accurate calorie burn for known BMR

Thanks to you, I am 11 years younger ... Great feeling ... !!!

But I do not quite get the second paragraph of your OP, when daily goal is below BMR ...
a) which BMR is it : the initial one or the more accurate estimate ?
b) when getting decent BMR estimate, is it different from the first point of the initial paragraph or should I use my new biological age?
c) is the final objective to input in HRM both a biological age and an hypothetical weight only to trick the built-in stats ?

Congrats on being younger, hopefully you are feeding that younger self. Remember when we always said we wished we had

Well, it's 2 sections doing 2 different things depending on your net eating level.

If you are net eating at or above estimated potential BMR, then it really has a chance of being that high, therefore your metabolism could be that of another age, comparing body comp BMR to weight/height BMR.

But if you are already net eating below any potential BMR, and plan on doing that still, then your real BMR is closer to what your body has left after exercise takes it's calories. Which means you will always have metabolism of someone older than you, at same height/weight.
The weight part to fool the HRM is needed because you can't put in an age over 90 I think. And if someone has suppressed their metabolism by 300-400 calories by eating low, you'd be around 150-200 yrs old mathematically.
I suppose you could actually max the age out, and adjust the height shorter to reflect a shorter metabolism.

So you do either part 1, or part 2 - not both.

No matter which part you do there though, doing the second half of the topic with getting correct MHR has a bigger bearing on calorie burn estimates.