I have a receipe for a 5 lb. standing rib roast but it's 10 lbs. How much do I have the change the cooking time to accomodate the bigger piece of meat?
Any time indicated in a recipe you are following should be used only as a general guideline. Cooking to temperature is much more accurate.
There are more variables than just weight (such as surface area) but I would use something like 1.5 times the cooking time as an estimate.
Sink a meat thermometer in that bad boy and pull him out of the heat before you reach your target doneness.
I would allow at least five degrees (probably more) for carryover cooking during the resting period.
The problem is -- it's actually an issue with surface to mass ratios, so when you have something that's twice the weight, if it's proportionally scaled, it's only the cube root of the original in terms of depth to the center of the meat. The result is that doubling can result in drying out the exterior before the center is done to your liking. For a rib roast it might just have more ribs, and so not be completely proportional.
You also have other issues that affect the time-to-cook: * What temperature it was at before it went into the oven -- how cold is your fridge? Did you give it an hour or so rest at room temperature before it went into the oven? * What degree of done-ness are you aiming for?
I'd recommend starting the roast earlier in the day, so that you can cook it to a given internal temperature, and then hold it in a warm oven.
To prevent drying out, you may need to tent it with foil while it's baking (which will reduce browning and/or flavor), add other moisture to the oven (but beware of basting, as opening the oven often causes more drying than the basting helps), or lowering the oven temperature slightly (which then causes it to take even more time).
If it were me ... I'd plan for:
- a worst case scenario of it taking 2x as long as called for
- roast it at 25°F / 10°C less than the recipe calls for UNLESS the recipe calls for starting it at once temp, then lowering the oven to a second temp (or visa-versa)
- roast it on a bed of onions and other root vegetables, to both add moisture to the oven and prevent the drippings from evaporating too quickly.
- trying to pull it from the oven about an hour before you planned to carve it, to allow it to rest.
If you don't have a probe thermometer or a leave-in meat thermometer that you can see through your oven's window, I'd hold off on taking the temperature until the normal recipe's cooking time has elapsed, and estimate when to next take the temperature from there (and possibly how to adjust the oven temperature to try to better hit your goal serving time).