I suspect a mathematical formula COULD work, but it would have to be so complicated that it wouldn't be any more useful for beginners than learning a chart.

I say the same thing every time the starting hands discussion pops up. And that is that ANY of the common starting hand charts that you see in poker books or websites is probably fine. Because, frankly, while it is important to play a relatively tight range of starting hands and to play them aggressively, the more important elements of pre-flop play are what other people are doing before you act and what your action will likely cause people to do after you act. In other words, the chart is just a baseline, and understanding whether the particular situation you are in justifies a DEVIATION from the chart is crucial to good pre-flop play.

So beginners should go ahead and learn a chart. But it is more important that they start watching tables and watching players and understanding their calling, raising, and folding ranges from various positions. Because the chart is just the starting point.