Lower the Z by clicking on -Z such that the A4 size paper just scraps the nozzle tip when moved between the nozzle tip and the print bed. If the paper is too tight to slide in, raise the nozzle. If the paper slides easily without any scarp with the tip feel, then lower the nozzle. You may want move Z by 0.1, 1 or 10 units based on the situation and your judgement (figure 6).
NOTE: While lowering the Z, the system might prevent you from lowering the Z-axis any further. In such case, enter the G-code M1012 in the manual insert G-code tab and you should be able to move the Z-axis further down.