Here's the calc I've used. I think it's based on the same Java app, but a little less advertising.
Snowboard Sizing Guide, Size Calculator
For shits and giggles just entered my GFs info. Spit out a 148 for a beginner, 150 for intermediate. We bought her a 149 a couple years ago and she's been thrilled with it.
I think in another year or two she'll be ready for a longer board if she gets more into freeriding and away from the freestyle end of things.