Sure if you at negativ 20 a Z-15 will move the spindle upwards.
But we are talking about to set the home position of our material, which is here at the left bottom if you look from top to the x,y axies.
Then I have to lower my z axies till the tool is scratching the surface.
In GRBL 1.1 is re-setting the x,y,z position to zero of all three axies by using this G28.1 - that is how it is working.
From this position I need to run the g-code, but the postcontroller always writing this Z-15 at a very first command which is not lifting the spindle.
As a GRBL G-CODE it needs to lift the spindle about 2 mm which would be a Z2 command and move the xy to the start position.
This is the whole issue here with my mill, and I still wonder in accordance of the postprocessor description, that the Z-15 command should lift the spindle, but -15 is a command to move it down.
Did I miss understood something?