recipe for chalk board paint

I love chalk paint but I have been making my own! I've been mixing 1/2 c of plaster of paris with 1/2 cup of water and stir until it's smooth then mix in with 1-1.5 cups of paint! I've now painted 2 dressers and a china hutch and they have all turned out great! I LOVE the mirror! I'm on the hunt for a cheap one now :)