meanwhile, what I did was added a macro to do it for me. And just had it name it something else.
[gcode_macro SCREWS_TILT_CALCULATE]
gcode:
rename_existing: _SCREWS_TILT_CALCULATE
G28
G1 Y100 X100 F7800
_SCREWS_TILT_CALCULATE
G28
#description: G-Code macro to home, move to center of bed, and run SCREWS_TILT_CALCULATE.1 and home again
# End Gcode
By renaming it with an _ hides it from mainsail/fluidd so both dont show up.
This would seem strange. Each SCREWS_TILT_CALCULATE run is a fully self-contained process and as such this approach as shown by @DaVinci10 should work.