I'm thinking, when I make myself a white "kitchen cake" to go with the chocolate Mothers Day cake I'm baking, of weighing out 2/3 of a box mix (for an 8x8 or 9x9 pan), and then saving the other 1/3 box to add to my next strawberry marble cake, to boost the volume.

I'm thinking thirds because, as I recall, one box calls for 3 eggs.

But how should I adjust the cooking time?