Welcome to DC Tony!
You might find that you'll have to add an extra 15 min. or so, to the total cooking time. This would be due to opening oven to rotate, etc, and loosing heat.
That having been said: you still take it out about 5 to 10 degrees below what you are aiming for. It will finish while resting. For medium rare it is 145 degrees, approx. 25/30 min. per lb/500 g.
Sounds like this isn't your first experience in the kitchen,
but just thought it worth mentioning.