See my reply to this post: Need TAZ help, please - #3 by TheVirtualTim
You’ll perform a test … marking the cold filament at 100mm before the point where it enters the top of the tool-head and make a 2nd mark 20mm further up (120mm), heat the extruder to printing temperature, then tell the extruder to feed 100mm worth of filament (following the instructions linked in that reply).
After the extrusion completes, you’ll measure how much filament actually extruded (vs. the 100mm that was supposed to extrude) to determine if it is over vs. under extruding.
I supply the math used to make the adjustment in that response.