What type of baker percentage should one use for brown sugar when compared to white sugar?
First, be aware that substituting brown for white sugar will result in a different product.  Brown sugar has molasses flavors, and absorbs water differently from white sugar, so both the taste and texture of the final baked goods will be different and no amount of adjusting quantities will change this.

With that in mind, 1 cup of packed brown sugar, weighing 220g, is equivalent to 1 cup of sifted white sugar, weighing 200g, according to the book Kitchen Companion.  So in your example recipe, you would swap the 2.5kg white sugar for 2.75kg brown sugar.