Ugh... I have a meat thermometer and I have yet to use it! I guess if I were doing that recipe, I may shoot for the median time... maybe 25 minutes? Then follow with the foil tenting/carving. It is really hard to say unless you really know your oven. Any other information you can offer? How close you will follow the recipe... past performance of your oven?
I would rather undercook it and then nuke it for a few minutes to bring it to the doness you are looking for. I hate using the microwave, but I have found it invaluable in situations like this. But it does require compromise, meaning you would have to cut into the meat to check the doneness.