For the most part it’s a good idea to use variable sizes like em or percentages when declaring font-size as opposed to using fixed px when a user resizes the text in a browser. Most browsers now resize px, but I found it to be better practice using em or percentages.

Most of the work we do uses the YUI-CSS library which has a handy chart for determining font-size using percentage when using their library. For those not using a grid/font library, Aloe Studios Blog has a small utility EmChart to help calculate ems from pixels.

It’s worth checking out and hopefully it will alleviate some of the headache in trying to determine how to use ems.