This question has been discussed and debated ad nauseam, with no results. There was even talk at one point about a "legal mandate" for financial institutions, in the order of fifty miles. The problem is, whether the recovery site is near or far, people still need to get there for it to be useful. I don't know where you're located, but in New York City on 9/11 all bridges and tunnels were closed. If your backup site was across the river in New Jersey, or even in a different borough of New York, you were pretty much prevented from getting to it. And if you had to fly there, you were obviously out of luck.
Actual distance will depend on two things: As much out of the same power grid as you can make it; and still accessible under emergency conditions in a time frame that makes it worthwhile. Remember, if you're in tornado, or hurricane, or earthquake country, your ability to get places may be just as curtailed as it was in NYC on 9/11. It can really help if employees live in different directions from your main site, and at least some are likely to have a way of getting to your recovery location even if others can't.
In short, you seem to be considering all the right things. But the only absolute regarding distance is that there is none. And in my opinion, there never should be.
This was first published in July 2005