I'm a big fan of nice typography (stopping you before you yell and scream about my last site design, I was experimenting!). One of the key pieces to having great typography is having a great measure, or as it's probably more commonly referred to, line length. For the English language, the ideal measure as I've been taught is generally between 2 and 3 times the length of our alphabet, including spaces. So, when I'm building and designing sites, one of the factors I take into consideration when choosing breakpoints is the text's measure. This will vary by font family, font weight, and font size. Because of this, I find I need to check my measure quite often.
I've built myself two tools to do just this. The first one is a .txt file where I can simply paste in my measure and visually see if it it's too large or too small. The second one is an Alfred Workflow for the new Alfred 2.0 (if you don't already use Alfred, you should really start, the basic version is free). For the Alfred workflow, I believe you need the Powerpack, which I think is worth it. The Measure workflow will allow you to type 'measure' in to Alfred, then paste in your copied line, and it'll notify you whether or not you've got an ideal measure. Hope you enjoy!