You can use relative by setting this option to 1:
DEV_OPTIONS: Development options
Note: This parameter is for advanced users
Bitmask of developer options. The meanings of the bit fields in this parameter may vary at any time. Developers should check the source code for current meaning
Bitmask
Bit Meaning
0 ADSBMavlinkProcessing
1 DevOptionVFR_HUDRelativeAlt