**I want to do a draft calculation to determine how much is the VOC in a paint formulation. But I don't know where to begin ?**

If you mean latex paint, then simply add all the VOCs reported from the vendors of your raw materials, adjusted to the amounts in your formula of course. For instance, if you buy a latex from vendor A, and they report 100 ppm VOCs, and this latex comprises 90% of your formula, then the latex contributes 90 ppm VOCs to your paint. Do the same for your other formula RMs. If your vendor refuses to supply you with this information, find another vendor.

Of course, if you make your own latex, you will have to demonstrate VOCs by laboratory measurement. If you are making oil based paints, then I wouldn't even try to calculate VOCs, it's better to just make laboratory measurements. There are a number of accepted VOC testing protocols, try ASTM to start.