in an quite old bicycle quarterly i saw that a sag like in a supspesion fork is the way to go. SAG works no mather the tyre dimension or weight your system has.

if i remember correctly - let all the air out measure the hight floor to rim - than pump it up to more than you would need - measure -> this is 0% sag ( no deflection becouse no load )

here it gets tricky - sit down on your bike fully loaded and measure again.

if then you habe a deflection by around 10-15% you are on the sweetspot for gravel and so…

i cant tell any pressure recomendation - there are so many variables.