If you don't know the weight of the bowl, then you can't subtract it from the batter to isolate the batter's weight. Instead, put the cake pans individually on the zeroed out scale and add equal amounts of batter to each until you've run out of batter (I'll start with 1lb (455 grams) of batter in each pan, then add a few more ounces to each, depending on how much batter I see left.)