Callibrating the Z axis

#1
So i after putting together the kit and getting a wire clip to successfully print i was quite happy. I printed a 20x20x20mm hollow cal cube with mostly good success. the bottom 10 layers or so dont look so great and as the cube grows in height it looks pretty perfect. No sags on the lid but the height of the cube is 18.3mm. A small bust i tried to print also is looking like it is somewhat squashed in the z axis. I will post some pictures later on if needed. But the general question is how to calibrate the z axis better?