LOL.... or always have a tub of baking powder in the store cupboard (I keep baking powder, cream of tartar and bicarbonate of soda in mine, as well as all sorts of different types of flour, just in case.....)
Here's a recipe for a sponge cake that uses no baking powder, so you can see the quantities (from
http://www.allcakes.info/cakes/sponge-cake-rec ipe.php)
LEMON SPONGE CAKE
6 eggs
1 cup sugar
2 tablespoons freshly squeezed lemon juice
zest of 1 lemon
1 cup flour
1/4 teaspoon salt
1/2 cup chopped pecans
Preheat oven to 325�F.
Beat yolks of eggs until thick and lemon colored. Gradually beat in the sugar, grated lemon rind, and juice. Fold in half of the stiffly beaten egg whites.
Cut in sifted flour and salt, add nuts, and remaining egg whites.
Bake in a slow oven for about one hour or until toothpick inserted in center of cake comes out nearly clean.