Going by a tip from this year's holiday issue of Fine Cooking, we have started weighing our cake batter with a kitchen scale. We weigh the bowl by itself before we begin and weigh it again after we've made the batter. Subtracting the bowl's weight, we know exactly how much our batter weighs.
Then it's a simple matter of dividing the batter between our cake pans - by weight. This works perfectly every time without any of the guesswork we've relied on before.
By the way, this is also a great trick to use for dividing bread dough, quick bread batter, or any other baked good that makes more than one loaf or cake.
Related: How to Make Your Own Cake Stand