Usaually figure 15-20 mins/ lb. at 350deg. You can find the weight by didividing the per pound weight into the total cost.
Preheat the oven to 450 deg then after placing the roast in for 15 mins reduce to 350 deg.
Keep the thermometer in the center of the roast away from the bone.
It's gonna be great.
Merry Christmas!