Actually, it's probably better if you don't do it on the bubble sheet because when you grade yourself, it is easier taking notes and putting comments next to your answers on a regular sheet of paper. You can't really do much on a bubble sheet.
I don't see how using just a left corner to number your questions and write answers wastes the page. You can write the explanations right next to them so you have the question reference to the explanation but do whatever you've been doing it, you seem set on the way you're doing it. I was just suggesting a different way.