Well, that does sound a lot like what Richard came up with above:

Once you know your destination, you are already there.

But... I take that comment to be somewhat of a negative statement. As in, you shouldn't know your destination, otherwise all of this rationality stuff is pointless. The phrase I recall (and perhaps what you're suggesting as well) was almost more of an encouragement -- like, "If you're endlessly deliberating, it will be more useful for you to head in the direction you're leaning vs. continuing to deliberate as if you don't know at all."