The 62.5% Trick

In May of 2004, Richard Rutter wrote a seminal piece in web typography in which he outlined a trick that helps make this math easier.

Because typical browsers default their text size to 16px, one can set the font-size value of the body element to 62.5%, effectively resetting the default to 10px.

body { font-size: 62.5% }

This resets the font sizes for the entire site so that 1.0em is the same as 10px.

Simply insert the font-size: 62.5% bit into your body styling and you're done.
Now you can use ems to sizes your fonts rather than pixels.

So your paragraph styles might look something like this:

p { font-size: 1.2em; line-height: 1.5em; }

Simple increments:

0.9em is the same as 9px
1.0em is the same as 10px
1.1em is the same as 11px
1.2em is the same as 12px

etc, etc.