Put the ham on a rack. You can line the roasting pan with foil for an easier cleanup. Cover the cut end of the ham with foil.
Score the ham in a diamond pattern, cutting just 1/4" into the surface in a diamond pattern.
Mix together the brown sugar, mustard, garlic, rosemary, and pepper and set aside until later when you use this as a glaze.
Bake the ham (without the glaze) for 1 hour or so, until the scoring pattern is raised a bit on the ham.
Put the glaze all over the ham and bake another 2 1/2 hours. Remove the foil from the cut portion of the ham, baste the ham with the drippings from the pan, and bake another half or so. You should count on about 30 minutes per pound at this temperature. If you're in a hurry, raise the temperature to 350 and bake about 20 minutes per pound. Most hams have baking directions on the labels so follow those if in doubt.