What is the relation between height, width, feed, over and under extrusion?

[Scratchbuild]

Hi there,

Newbie here. As the subject asks I need to understand the relation because I am struggling to control my over/under extrusion and my dimensions. The reason I am asking is that this weekend I re-levelled my bed, although I have auto bed levelling. By doing this, the height of the table might have changed and therefore the nozzle height with it.

It now prints the bottom layer with too a small a nozzle height and the upper layers looks over extruded. So I need to know the relation before I just blindly go and change the parameters.

Please give me an explanation of what changes when I change the Feed Rate for instance.

Regards

Nic