> Should I set X min positionX_MIN_POS and Y min positionY_MIN_POS to -28,-4 respectively?
Not relevant, but it is convenient if bed start with 0,0 so choosing min pos such that this is the case is a good idea. Otherwise you need to configure all beds to start at 28/4
>Are the extruder offsets useful for this ?
No. They are only useful if you have more than one extruder and then it should be offset to reference extruder.
