How do you work out your yearly holiday pay?

I've just received my vacation money and I am a little underwhelmed by the amount.
Without going into too much detail, I have been working here 4 years now, the first few years in my first job I got a decent amount each June, but this year I received much less than I had expected.
(I'm in my new job since March 2011 if that makes any difference at all)

Any help or info on how to work out what I should get would be appreciated.

Cheers.

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