Hi - It's useful to know some general figures - like the calories in a gram of fat (9) and in a gram of protein (4-5). The problem is that a gram of anything will include water content. If you feel the need to count calories - just work with approx figures - being a little bit out wont do you any harm (I appreciate there are some people on this web site that might faint at that last statement). Gradually, instinct will kick in and you will just know. Portion size is just as good as any at making sure you are eating appropriately.
Generally, people do not expend the same amount of calories every day (hot days/cold days) longer working days, walking because you missed the bus etc, so don't loose sleep over precision.