Unfortunately, there is no simple formula for calculating a general risk of ruin based on actual trades. It is a nasty mathematical snarl that needs to be solved with a computer, only simple and fairly unrealistic cases such as the symmetric risk of ruin and Kelly risk of ruin can actually be reduced to simple arithmetic.

Fortunately, you don't need to worry about the formula, Futures Magazine has an article on their web site describing the balance between risk of ruin and Optimal f, which you can read at http://www.futuresmag.com/industry/references/roberts995a.html.

The best bit is that the article links to the downloads page where a free program can be downloaded that enables you to enter the returns from a series of trades. The program will then prepare a chart with Optimal f on one axis and risk of ruin on the other. This will help you understand the relationship between trade size, return and risk.