Z hits homing switch at project start

I am not understanding something here. Everything has been fine until today. Started the machine, homed all, secured my material, set X , Y, Z Hights. Load the project, hit start and the Z raises up and hits the home switch. It use to stop just before the home switch.

I tried moving the switch up and down, I have tried setting my Z travel higher/lower. nothing seems to change. At the project start, my Z raises up and hits the home switch.

The Z height is set according to what 2 points?
Does it matter where the home switch is placed?
The Bit raise at start is measured from what 2 points?

Thanks

EDIT:
Never mind, I figured it out - somehow my Z got set to 0 in the post process.
But would still like to know more of how to properly set the Z height if there is a guide I can look up.

 

Awsome, thanks