Physic Material friction mathematical formula

What's the mathematical relation to calculate the friction, ie. what formula to put the friction values of a Physic Material into?

My attempts at figuring it out myself were without success (though it might be that I just used wrong values for my calculations) using my elementary-school-level physics knowledge. (did use google too but it just seems like unity's friction values capped into the 0-1 range are different)

{bump??? - desperately trying to figure it out for himself}