I am now very happy with how the printer is working and it is producing some good parts. However, these parts are still not quite right dimensionally and this is where I am still confused as to how the software is working. When the 'Calibrate' screen is selected some values from a previous attempt are showing. Should these be reset to zero before calculating the current values?

Also, barely visible next to the Z value it says 'Keep Zero'. Unfortunately I changed this value after the first print because the Z dimension was slightly out but because the 'Keep Zero' message is so feint I missed it. Why are we allowed to change the value if we have to keep it at Zero? How do I now reset this to zero?