Could use some help with step calculations

I have an Anycubic Kossel Linear Plus, with Marlin 1.1.9 loaded. The steps/mm is now calculated as follows:

// variables to calculate steps
#define XYZ_FULL_STEPS_PER_ROTATION 200 // These are the setting for the stock Stepper Motors, pulleys and Belts on the AnyCubic Kossel
#define XYZ_MICROSTEPS 16
#define XYZ_BELT_PITCH 2
#define XYZ_PULLEY_TEETH 20

// delta speeds must be the same on xyz
#define DEFAULT_XYZ_STEPS_PER_UNIT ((XYZ_FULL_STEPS_PER_ROTATION) * (XYZ_MICROSTEPS) / double(XYZ_BELT_PITCH) / double(XYZ_PULLEY_TEETH))
#define DEFAULT_AXIS_STEPS_PER_UNIT   { DEFAULT_XYZ_STEPS_PER_UNIT, DEFAULT_XYZ_STEPS_PER_UNIT, DEFAULT_XYZ_STEPS_PER_UNIT, 98 }  // default steps per unit for Kossel (GT2, 20 tooth)

Unfortunately my parts are coming out slightly smaller than intended. especially in X and Y (probably Z as well, as it's a delta) - to get the exact size I have increase the scale in the slicer by 1.012.

My question is - what is the correct way to get this enlargement factor into configuration.h?

Or do I have another problem that I've missed?

Thanks,
David