I reiterate, you've got to use a thermometer. Cooking time is dependent on surface-to-mass ratio, deepest point from any surface, circulation of heat... Lots of things more complicated than weight and temperature.
A probe thermometer would be best, but any meat thermometer jabbed into the deepest, thickest part of the roast is better than none at all. When that sucker hits 120 degrees, pull the meat, and wrap it in foil to rest it before carving.