The actual formula for how the weight of ratings work was released once, then hidden again, then reworked, then the post was probably lost to the ages. It is dark knowledge held only by the elder scribes and people who have the power to look at site scripts and shit.
However, if you'd like my rough estimation, if guests have any effect on ratings, it's very little. You only tend to see impact with people in the 500-1000 points range and higher.