It's a 'as good as any' because there's no exact science in calculating these figures. Here's a link to an estimation by a number of aviation sources across Europe:
http://www.eraa.org/intranet/documen...5fastfacts.pdf
They estimate 4,000 jobs per 1 million passengers which actually makes Ryanair's 2,500 for 2.5 million passengers look like an under-estimation! I'm not really sure why you're bringing currency exchange rates into this.