Good info thank you - confirmed my suspicions
On this occasion I was trying to get the most accurate position so used the DTI to be extra sure!
I do normally write down the machine coordinates of the work origin as I've never got the save fixture on mach3 exit to work. Maybe I'm doing it wrong but when I switch mach3 on again the numbers in the DRO appear random.
So I home the machine, MDI to the previous coordinates then zero the DROs.