You must make sure that in the g54 coordinate edit screen Z is zero before you set all your tool offsets. Set all your tool offsets (H's) first then change the G54 Z coordinate. If you run a program then decide to add another tool, change your G54 Z back to zero then the set the new tool offset (H), then change your G54 Z. I always use a 2" tool offset guage so my G54 Z is always zero when setting tools and -2.0 when cutting.