I stick to this basic measurement for all my sponge cakes, pineapple upside-down cake, lemon cake, coffee cake- you get the idea.
6oz (175g) self raising flour
6oz (175g) caster sugar
6oz (175g) soft magerine (I use clover or Willow)
1.5 teaspoons baking powder
vanilla extract - a few drops
around 100ml of hot water
good instant coffee (generous amount of instant coffee powder dissolved in the hot water)
3 eggs
sift all the dry ingredients together, add the wet ingredients and either using a hand blender or hand whisk mix all of it together for a smooth, thickish batter. Don't overdo it otherwise you'll knock the air out, don't worry about a few lumps.
This can be used to make a Victoria sandwich (batter in 2 sandwich tins) or all the batter in a big enough cake tin. I bake on middle shelf of pre heated oven gas mark 5 for about 50 minutes, just check with a skewer or knife to see if it's done