I have a few standard portions that I almost invariably stick to. It makes costing so much easier.
rice, pasta, spaghetti - 50g
oats for porridge - 50g
bread for toast, with soup, etc - 2 slices
baked beans - 1/3 can
So today's beans on toast is actually 1/3 tin of beans and two slices of my homemade toast. :-)