Note the related concept of a modular scale (http://alistapart.com/article/more-meaningful-typography http://alistapart.com/article/more-meaningful-typography,

Note the related concept of a modular scale (http://alistapart.com/article/more-meaningful-typography http://alistapart.com/article/more-meaningful-typography, http://www.modularscale.com/ http://www.modularscale.com/) which offers a bit more flexibility (i. e. if everything has to be a multiple of 18px, your H1s are going to be REALLY BIG.