A small tip I found useful from Trent Walton’s blog on how to easily set a readable font size and layout breakpoints with media queries that respect the lines length of 45 to 75 characters:

dropping some lorem text into the layout with two asterisks. The first marks the 45th character, and the second marks the 75th

It is then very easy to resize the browser or change size in Firefox’s responsive view and see whether it displays the correct line length at any resolution. If at some point both asterisks are on the same line then it is an indication that either the page needs a new layout breakpoint or adjusted font-size.