Its about 20 years since I last owned a Protractor
Me too, but it's not exactly difficult to draw a diagonal-ish line. Quite impressed I got exactly the same as what @pacegod suggested above.
BTW, ideal screen angle depends a lot on how far you are from them. With 27" monitors, 45 degrees works if your eyes are about 750mm from the centre one. If you're further away they'll need to be slightly flatter and therefore a bit wider overall.