EXCUSE THE SHOUTING BUT I WANT TO BE EMPHATIC
DO NOT TURN OFF H & T AGREEMENT!!!!! THAT WAY LIES THE POTENTIAL FOR SERIOUS CRASHES.
Okay now I will calm down.
Easiest way is G51 Z(some value).
Set your tool offsets to a point somewhere above your highest work location then you adjust downwards by using G51 Z-nn.nnnn.
The reason i say "above your highest work location" is because then you have to use negative Z values. If you make an error and enter a positive value this means the tool stays way too high.
However if you set things up so that the Z shifts have to be positive...and make an error the tool goes way too low probably with disatrous consequences.