You must start from the ingredients list on the cake mix box. You can also use the nutritional info box to tell you how much fat, sugar, etc there is in the full box of mix.
Then you have to recalculate with your extra ingredients. And because the cake mix has some additives, all of that will appear on your final label.
No it's not simple...I guess that's one reason that a lot of people are suddenly asking for scratch recipes. I expect that sooner or later there will be an app where you plug in the brand name, the addins, and you get the right label.