How close should my fan be without a shroud?

Now, I DO realize that taking it to a radiator shop and having them service the core is best, but have you guys had any success cleaning them yourselves? I mentioned before that I used CLR with little success, but then again it was very diluted and only soaked for an hour.
When they are stopped up that bad, the only way to really do it right is remove the top and bottom of the tanks and rod it out.