Question from a newbie

I never counted calories only carbs... I can't stick to low carb anymore so I'm trying calorie counting.

Question: When I cook something but don't eat the entire meal, how do I calculate what I consumed?  Example: oatmeal-I cooked 1 cup of oatmeal and added 5 tsp of brown sugar.  If I don't eat all of what I cooked then I didn't eat all the sugar, how do I calculate that into my daily log?  Is there a formula?

Unless you measured how much you ate, I would just estimate on the portion consumed.  I don't know of any formula for this.  You should be able to get close enough for your log.  In fact, labels can be "off" by like 20% so you don't even know the true cals on everything you buy.