In heavy skillet, slowly heat 3/4 cup sugar until melted and caramel-colored. Pour into 5-cup baking dish and carefully swirl to line dish with caramel. Set aside. Beat egg yolks with 1/2 cup sugar. Add evaporated milk and rum to taste and beat well. Turn custard into prepared baking dish. Place dish in pan of hot water and bake at 350F about 1 hour, or until knife inserted in center comes out clean. Cool thoroughly at room temperature. Unmold on serving plate.