My father worked for many years at a sugar factory, and I can tell you there is no such thing as a natural "brown sugar". Brown sugar is simply post-production white sugar with molasses added. As you make your recipe, use slightly more than the called for amount of brown sugar--maybe an extra teaspoon or two, and then add molasses. If the recipe calls for light brown sugar, add a little molasses. Dark brown sugar? Just add more molasses.
Also, because you're storing the components separately, your "brown sugar" never gets hard as a rock, because you mix it at the time of use. Why people will spend twice as much to buy brown sugar, when they probably have both white sugar and molasses already on hand, escapes me. Homemade brown sugar is less expensive, softer and so easy to to make. - Shirley A.