If you are using the MedHelp app My Diet Diary, then it would be best to update your weight whenever you weigh to get the best results. Or alternatively, you could buy one of the devices like Withings scale to have that information synched automatically into your app/tracker (
http://www.medhelp.org/land/withings-scale).
My Diet Diary app gives you a guidance on how many approximate calories to eat to get the desired result, but don't worry about being too stringent all of the time. Take them if you need them. Just eat less sugar, eat more real food, and cut out some of the worst foods (soda, low-fat foods as they have exceedingly more sugar in general, chips, etc)