One thing I don't understand (which would definitely help me to use this function better) is how exactly optical margin alignment really works and what exactly is measured in pt, or when to set what. do the pt relate to the spacing at the edge for alignment or to the font size or something else entirely?
In my experience the best way to set the optical margin alignment is to set it at the size of the body text. It allows characters to extend over the margins, but the amout is depending on the shape of the character. If you zoom in on text you will see on normal text that round characters like o u and sharp characters like v w normally extend below the baseline. A similar thing happens at the Cap height if you set a guide on the flat of Hor N. These extensions are so that the baseline is optically correct.
In a text like a novel with indents and much dialog you will notice that without optical alignment the indents that start with a quotation mark or an EM-dash appear larger than normal indents. Optical alignment helps to make such indents appear more regular/balanced.
The right margins are also affected but this is mostly seen if you have justified text (avoid full justified text).
There are ofcourse situations that automatic optical margins cannot resolve and you may want to kern manually.