A good translation would depend on the subtext, and also the relationship of the speaker and listener. One can say "If you think about it..." to present a conclusion that the speaker has already thought of, with the (somewhat demeaning) implication that the listener hasn't thought through the issue enough to arrive at the same obvious conclusion.
But it can also be used in a more collaborative way, as if the speaker is thinking about it right now and coming to a conclusion on the spot. What popped into my head first was 考えてみると... The nuance in Japanese matches the second situation more closely, in that you are the one who is thinking about it and simply presenting that conclusion rather than demanding that the listener think about it.
ALC has a nice collection of sample sentences including this phrase.
I also referenced a book on tactful speech in Japanese geared towards professionals for some examples of looser translations which achieve the same result. (できる人の物の言い方大全) I came across the phrase お言葉を返すようですが... which is a tactful way of disagreeing with someone in a position of respect.
I hope this helps! I'm very interested in hearing some other perspectives on it.