Your math is correct for the mercury to pounds and so forth.
The math for the force is correct as well.
As I understand it, braking is done on decelleration and therefore the vacuum is highest. There is a check valve in line with the diaphram ( on my old Ford there was ) so there is assist most of the time.
When the pedal is worked often with little or no vacuum, the power assist is depleted.
I am not sure, but I think the ratio of 6:1 may be correct. Given this, it would take 10 pounds of force to get an equivalent of 75 pounds.
I am sure this is why some vehicles will stop by gently pressing on the pedal.
What is your quest?
