Sit n Go Standard Deviation Question

[Timer]

I have an online sit n go standard deviation question. Let's say you cash 40% of the time, and your cashes are equally divided between first second and third place. This means that you average about $7 for every $30 entry. How would you compute your standard deviation and therefore your bankroll requirements? And once you have these calculations is it a linear relationship to determine SD and BR for the 30, 50, 100 and 200 sit n goes?

Thanks