Remove the ham from the refrigerator 1-2 hours before it needs to go into the oven. This will allow the meat to come a bit closer to room temperature. Place the ham on a large cutting board and score the ham with cuts, in a diamond pattern. The end result should resemble a checkerboard. This process allows the glaze to penetrate deeper into the ham. Preheat the oven to 325°. Line a large roasting pan with foil and place the ham cut side down in the pan. Roast 45-50 minutes. While the ham begins roasting, combine the sugar, vinegar and mustard in a small cup or bowl. Set aside. Remove the ham from the oven and baste generously with the glaze, using about 1/4 of the glaze. Continue to roast the ham until it is deep brown and glazed, brushing the ham with more glaze every 10 minutes or so. Use an average of 10 minutes cooking time per pound of ham. Transfer the ham to a large platter and let rest 10-15 minutes prior to slicing.