Last years roast was very tasty but well done. We had 24 people to feed and they all brought dishes that needed to be kept warm or cooked. The meat got tired of the wait.
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I would use an inserted thermometer with a remote in each so you can monitor the internal temperature. It's temperature not time.
Cook time should be as if they are separate. Absolutely use a meat thermometer and it may be an instant reading that you do not leave it in. With the investment that you have in the beef you must use a thermometer...in my opinion.