I'm trying to place four circles inside a rectangle, all with the same margin from the corner edges and although I can do this using guides or some calculations, I was wondering if there is a better way of doing this.
For example, can I select one of the circles and set it 20mm away from the bottom-corner edges of the rectangle, simply by choosing the edge line and entering the distance in an input field?
Of course you can do that. In this example, placing the circle 20 mm up/right from the bottom/left corner of the rectangle:
X Value field and type +20 → EnterY Value field and type -20 → Enter
No, illustrator is a direct modeller. It has no understanding of relationships of objects.
However there is a easy way to do this. Offset the square by -20 mm then copy-drag the circle form center of the circle object by holding alt. and snap to the corcer points of of offset rectangle.
PS: there are cad apps thst can do this easily but they are not direct modelers.
Illustrator already tells you the distance you've moved an object from its initial position. I would either use guides, as you mentioned, or use the built-in system.
For instance, you can see that I dragged the ellipse below 30px from the top of the rectangle. Just change your units in AI to mm instead of px:
