Finally managed to figure out how to create custom sizing for this. Not really intuitive to be honest, but I suppose you have no other way around.
For example, it would be better if each value would mean the distance from it's own margin, just like in CSS. Margin 20 20 80 80 would go clockwise and thus you have the amount of space relative to the edges of the screen. Are you forced by some circumstances to go from top left edge of the screen? Thanks!