Need help explaining text sizing methodology...

I've created a site using this method:

http://24ways.org/2006/compose-to-a-vertical-rhythm

I'm having difficulties resizing the text on the page...specifically, I don't understand the section entitled Variations in Text Size. Can someone explain with a couple examples in both directions?

Say your main body text is 12 pixels and its leading (CSS line-height) is 18 pixels. You want a heading in a bigger text size. You choose 24 pixels. You need to give it more room to comfortably breathe (read: not let text overlap it), so you need to adjust the leading. The new leading must be a multiple of 18 pixels, because otherwise you will throw off the vertical rhythm. 36 pixels is appropriate (:

You can do the same with padding &c, as long as the total computed height of the element is a multiple of 18.