Perhaps you want to look into the Measurement Log! I have not used it but there seems to be some note about the rulers and the tools measurements units. The rylers around the document are the same Adobe's preset but the Ruler tool can measure with your custom setting which show in the tool option bar. When use measurement is checked and the measurements can be recorder in the log
I don't think adobe could have screwed up the ruler function any worse. It's so underwhelming and non-intuitive that it reminds me of the windows paint program on Window 3.0. So many great finctions and potential being left on the table.