You don't really, the adjustment on 3d print is normally a function of the extrusion amount - too much extrude will make things bigger externally and smaller internally.
I use Slic3r for my 3d code generation and it has adjustments for the extrude width etc and must be calibrated every time a new roll of filament is added as they are all different and thicker filament=more extrude, naturally if you have a granule feeder then this will not apply.
Mach just does what the code tells it