Math captcha settings
The tab 'Math' on Administer » Site configuration » Captcha (admin/settings/mycaptcha/math) doesn't contain a whole lot of settings. The only configurable option right now is the maximum answer of the math captcha. This is a measure of the difficulty of the captcha for humans (at least for those who can't find Calculator in their Start menu). The higher the maximum answer, the lower the chance that a bot guesses the right answer.
Note: It would be fairly easy to write a bot that could solve this kind of captcha.