Front does 80% that’s why you run bigger rotors up front.
That’s a completely different question … and a bit sledgehammer and nut.
If you just want to bleed some speed then your only using 10% of the brake power …
only using the rear for speed modulation and only bringing the front in for emergency stopping is the proper way while I say mainly using the front with rear for added support is the right way – In general of course as specific trail elements might see no front brake use at all.
I think the last part is most relevant … also of course depending what tyres you’re running.
I tend to apply more back brake on loose, especially if its a buildup to a corner but that of course doesn’t mean there is less front braking taking place … just I’m applying more pressure.
Otherwise I use both more or less equally in terms of pressure for slowing down a bit (again meaning more actual braking is from the front) and might modulate a bit if for example the back was skipping or the front starts slipping … or move my weight further back …Of course if I want to shed a load of speed really quickly then I’ll be as far back and heels down and give it lots of front…
So I’d say a simple rule either way is not really realistic and trail conditions and where/what are the deciding factor and as above boiling brakes are a factor of dragging brakes so its more a case of letting them off between than applying them.
If you’re racing then more front = faster braking … meaning less loss of time but that also carries inherent risks that might be worth an extra second racing but not on holiday.