In order to change the extruder from its current extrusion of 150mm to 100 it has to be reduced by 1.3 or 50mm
multiplying by 2/3rds or .666 gives us 400 + .6666 = 266.664


#define DEFAULT_AXIS_STEPS_PER_UNIT {80, 80, 400, 266.664}

after changing the settings you need to read the values from firmware.
M502
and Save them to memory as the new defaults.
M500