This is a 20mm x 20mm x 20mm cube with an open top and 0.8mm walls. I use it to calibrate the extrusion multiplier for my slicer for new spools. First set the multiplier to 1. Then change the extrusion width to Manual and set it to 0.4 (for 0.4mm nozzles) then print the cube. Measure the walls and take an average of all 4 sides. Then divide 0.8 by this value and you'll have your exact extrusion multiplier.
The infill percentage does not really matter. Set it to whatever you like but don't make it zero.