Summary

I was suspecting that my 3D printer was printing things 2% too small. As if the steps-per-mm for X and Y were too small.

This is hardly possible because some pretty reliable calculations can give you that number to many decimal places....

Instructions

Print this and measure the distance from the top of the L to the part sticking out at the bottom.

Oh! If, after printing this, you're still not satisfied that your movement calibration is accurate, you can rotate it 90 degrees for testing in the other axis. (This would make sense on a reprap which has different mechanisms for X and Y movement. )

The cutout near the corner of the L is there to ensure that both measurement surfaces are approached from the same side. Any hysteresis should cancel out like this. If you print it at 1.0 scale, it should come out at 100mm.

(I've improved it slightly since printing it, so you'll get the improved version over the one in the photograph.)

I've printed it with 30% fill, 3 layers of bottom and top thickness at 0.2mm layer height.

length calibration stick.