Try to get the perfect font size an other way (with a setting rationFont to adjust un function of your font configuration).

Example:
http://jsfiddle.net/CDLXE/8/
ratioFont of 2.1 is net bad for Oswald with the default letter spacing (firefox 29 in the test in JSFiddle)
rationFont of 1.72 is not bad for Helvetica Newe...