Rampant moral philosphy geekism

An utilitarian argument not to use pure utilitarianism as the basis for ones decision:

Pure utilitarianism has a tendency of high error rate due to the standard problems with using the ends to justify the means (such as the cases where one is mistaken about the end result of the means), leading to lower net happiness or other criteria of good. Thus, it is more utilitarian to temper ones utilitarianism with other moral philosophies than to employ pure utilitarianism.

I enjoyed writing that little paradoxical bit. And writing this has made it evident that I'm not an utilitarian, at least not precisely.

Why do I sometimes feel like I do moral calculus where many others use moral basic arithmetics or at best moral algebra?