Any Morbidelli specialist here??? Need help on a 504 author

I need help setting the z 0.0 and the tool lengths.
Ive been running on some test cuts with some old tool defs and had it working fine, but the tool lengths figures do seem of any relevance to the physical length of the tool so I am unsure of where the tool length is set from?
Are the lengths from the bed / rail face? If yes where are they measured to?
I also skimmed the surface rails to produce a nice new flat face, how do I adjust the z 0.0 to suit? I know I can use an offset, but im not sure if the had been skimmed before so my z datum could be anywhere?