Is Utilitarianism Arbitrary?

Based on what we've learned this past week I think perhaps the greatest accusation that can be levied against utilitarianism is that it can't always provide a solution in a given situation. Utilitarianism suggests that rationality be used to make a decision, which comes with the implication that multiple solutions can be determined for what action one should do, as long as they all seem reasonable through use of rationality. The one aspect of utilitarianism that always applies is that one's actions should be oriented towards giving the greatest amount of happiness to the greatest number of people and the least unhappiness to the smallest possible number, but since consequences are relevant to the action's justice in utilitarian philosophy, then if one's rationality in a making a decision results in less happiness than if a different decision had been made, we can say that that action was less just than it should have been, based on other hypothetical consequences from that action. From this, we can accuse an action with good intent and an application of rationality as having been just one of many choices with different consequences, and from there say that the decision was an arbitrary one.
However, I think a similar argument can be levied against Kant's deontology, because it too requires use of rationality in decision-making. As discussed in class today, Kant doesn't see rationality as being different from person to person. For example, 1 = 1 is a conclusion any person would presumably make through use of their rationality, and in this light it does not depend on who the person is or their situation. However, for situations more complex than stating the axiom that 1 = 1, I think the supposed universality of rational choices becomes less evident. For example, when one performs a moral action that requires using rationality to determine what choice is best based on that person's philosophy, how would one go about proving that that person made the same decision as any other rational person thinking rationally? There is no written in stone standard to consult for such a proof, unlike with an example such as the axiom of identity mentioned earlier. If it can't be proven with certainty, then some criteria must be adopted for one to determine the decision's degree of rationality. This criteria will come with its own posits and suppositions, and could itself be accused of having been made based on whim or random choice. As such, it seems to me that even deontology can't escape some arbitrariness. What do you all think? Could the decisions made from the logic of both deontology and utilitarianism be called arbitrary?