Its the flour that makes the cakes rise, I think. I always add half a teaspoon of baking powder (even with SR flour) it makes my cakes extra light.
I remembe when we were being taught to bake at school - we were told that, if you didn't have any weights for your scales, you could use eggs as a weight. It really does work! If you don't fancy using that method, just remember that for each egg you use, you must use 2oz of flour, butter and sugar. There are only two of us and I find that a one egg mix makes just enough batter for about 6 cupcakes.
Regarding yorkshire pudding batter, I too always struggled to make them right. Then I heard someone on the radio saying that all the ingredients should be equal by volume - not weight. So now I put two ramekin dishes together and break one egg into one. Then I put plain flour into the other till its the right amount, then tip the egg into a bowl. Then measure out the milk the same. Then tip both the milk and flour into the bowl, add a pinch of salt and whisk like made. Get the oil smoking hot before you put them in the oven - I promise you they will rise so much you can't put them near the top of the oven or they will stick! Again - if you want more, or larger yorkshires, just use two eggs in the same way!