Usually the guidelines will be on the side of the packet if you are starting with baby rice, powdered porridge etc.... I was told that a babies stomach is the size of her palm, I try an dremember this when I am measuring out food.
When I began weaning, I aired on the side of caution and probably underfed rather than over fed. I was happy with this though as I didn't want Lucy to drink any less milk then she was before... this way it took a while to drop any milk bottles...
good luck xx