I put 1 cup of water in a single layer box recipe that only called for 1/2 cup. How much cake flour do I need to balance out the water?
Unfortunately, the box of cake mix has much more than just flour in it (dried milk, sugar, leaveners, flavoring etc...)... and it may not even use cake flour.
Your best bet will be to buy a second box of the same exact cake mix and double the remaining ingredients as well, so if the came mix calls for 1/2 cup water and an egg, you'll need to add a second egg.
Now, it's possible to come up with a list of ingredients you will need to add to save yourself a trip to the store but we'd probably need to know which cake mix you used to start with.