Light brown sugar = 1 cup granulated sugar + 1 tablespoon molasses, dark brown has 2 tablespoons molasses. So if you need dark brown just add more molasses. I don't bother with the mess of measuring out brown sugar. If a recipe calls for one cup brown sugar I put in one cup granulated and 1-2 tablespoons molasses.
Merry Christmas! This is a link to one on amazon. It's not the same one as I have, but I found mine at an estate sale. KitchenArt Pro 2 Cup Adjust-A-Cup, Satin a.co/d/3QXvi7W