It sounds perverse, but even percentages need to change.
In a browser, margins of up to 10% can be nice. (Look at the present page.*) Drop down to tablet size and you're wasting real estate if you take that much horizontal space out of action; continue down to phone size and you'll want hardly any margin at all. It may be relevant that with smaller devices-- tablet or phone --there is no window, but the device itself has a physical "margin" outside the text area. This becomes part of the viewing experience.
When setting font sizes, 1em = 100%. So you can use whichever form is easiest for you to read and remember.
* Got a nasty feeling I've looked this up before and forgot. The entire page is a table ::cough-cough:: with width of 85%, meaning that the side margins are 7.5%.