Mesh Bed Levelling (no replies)

Hello everybody!

I am using manual Mesh Bed Levelling for my Tronxy X5S and my I3 Mega which works OK.
I have a question though.
The various Z offsets that are collected are added/subtracted from the Z zero (home) position?
Which means that in any case I first have to home Z, level the bed with the screws and then do the manual mesh levelling sequence?

And what if I use a proximity/touch sensor for auto levelling? What is the relation to the sensed distance and the actual nozzle height?

Thanks a million!

Giorgos.