What he typed:
percentField.rawValue = profitField.rawValue / costField.rawValue;
This assumes that the 'name' (under the 'Binding' tab on the object palette) for the numeric field used to enter the cost is "costField", and the one used to enter the profit is named "profitField".
And then that the name for the result is "percentField"
If you don't mind posting your email, I'll send you a sample.