Utilitarianism

Utilitarianism is the method of selecting right thing to maximise the total utility. Basically, every situation is probabilitical, so actually you must choose most probably maximising way. Id est, you must choose in the situation. Utilitarianism itself is consequensism, but it is the greedy-algorithmic version.
Then, here is the topic: actually you are very hard to measure the whole utility and make decision with your consumption of utility in real situation if you are utilitarian. And, this is right according to the above. However, if real situation is far different from what you assumed, which is right?