You might be better off learning about G50 and work offset shifts
Increment the whole work offset Z #variable position.
Screw around with tool length offsets is subject to a whole new set of "what the hell happened there"
You might beable to write to the tool length variable then call it with an H offset but resetting it and knowing where your at any one time is going to be suspect too.
Being new to this I advise to learn the basic before you crash, burn then