Welcome to the forum!!!
I realize that standards and regulations vary from country to country, but your weight should be stable prior to the tummy tuck. A tummy tuck should NOT be used as a means to lose weight. It should be used once your goal weight has been reached.
Are you getting lipo as well? How tall are you? Are you also getting a panniculectomy? These are important questions.
The amount of weight you will lose post op will depend on how much fat and skin are being removed. This number varies from person to person.