I rely heavily on the meat thermometer when I cook one. That way I don't have to worry about the math as much. I have a thermometer with the remote readout, so I can set it to beep when it gets to a certain temperature.
I only cook one about once each year, usually on New Years Day. If we don't do one on New Years, we usually do one for my folks anniversary at the end of January, because its my Mom's favorite.
Like Kevin says, I do cook it at high heat (450)for the first 20-30 minutes, then I turn the oven down to 325. I cook it until the meat thermometer tells me the center is at 120, this is usually about an hour +/-, depending upon the size. Then I take it out of the oven and cover it (I got a really good roasting pan with a tight sealing lid, but before that I just used foil, and sometimes the roast is bigger than the pan, so then I just use foil), then let it set until the thermometer says 130-135. That is still pretty pink in the middle, you might want to let it set until 140-145 depending upon how done you want it. Remember you can always cook it more, but you can't go backwards once it gets past pink.
