I think the main problem with trying to be 100% rational is that even if you could predict the outcome of your decisions perfectly, the question of which outcom

I think the main problem with trying to be 100% rational is that even if you could predict the outcome of your decisions perfectly, the question of which outcome is "best" is inherently not a rational one. First you need to have some goal in mind, then (maybe) you can apply logic and reason to achieve that goal. I think a lot of political disagreements aren't really about logical arguments at all, but rather differences in opinion over relative priority of some ideals that are all important. There isn't always an objective right answer.