I tend not to buy brown sugar for most cooking. I find that it gets hard in the box and is difficult to mix with other ingredients or to sift. I also suspect that brown sugar has gotten less brown than it used to be. Brown sugar is simply what white sugar was before all the molasses was removed. By mixing white sugar with molasses, I can control the “brownness” by mixing my own.
I think that a cup of white sugar weighs about 7 oz. and that a cup of brown sugar weighs about 7 ½ oz. If I need a cup of light brown sugar, I substitute 7 oz of white sugar plus 1 tablespoon of molasses. If I need a cup of dark brown sugar, I substitute 7 oz of white sugar plus 2 tablespoons of molasses. The molasses should be called something like full-flavor, and not light or blackstrap molasses, which are different products, and will affect the interaction with the baking soda. If I am making something sweet, often the butter is creamed with the sugar. In that case, I cream the butter with the white sugar. I then add the molasses and beat, scraping down the bowl until things are combined.