An em is a relative measurement so yes there should be about the same relative amount of space between lines. The best way to understand it is to try a lot of different values and units out and see how it looks. You can try it here:
http://www.w3schools.com/css/tryit.a...-height_number
You can put your own font-size in the box too, just remember to put a semicolon at the end of it.
That's how it renders in HTML, no guarantee that it will be the same on the reader (but in theory it should be).
eP