starting food!

My son is 4 months old and started food today! But the only problem is I don't wanna start wasting food and everything else. Tomorrow morning I plan on giving him his cereal with his formula in a bowl (as directed by DR) but I usually give him 5oz bottle with 1tbsp cereal .... I don't wanna give him that much because I know he won't finish it especially since he's just staring and I don't wanna waste basically a whole bottle. My question is how much cereal can I put for 2oz?