No there is no certain formula to adjust the paging file according to your RAM size. Actually the page file size don't affect the PC performance in a linear way, which means that as you increase the page file size, don't expect the performance to be increased by the same rate of increase. Your page file size comes to what is called saturation in which any more space added is just a waste and gives nothing more to performance.
The typical page file size is around 500 MB, but it's ok to up this to around 1 GB or so, more than that, I don't think you are going to get a sound effect in your performance.