Newbie and breast feeding - help!

Hello

Ive just joined today and ive lost almost a stone on my own in the last 6 weeks but felt i needed to some extra help as motivation is lagging a bit! I want to loose another 3 1/2 stone as im getting married in sept. But i have an 11 week old son who im fully breast feeding and im not sure how many extra calories i should be eating as myfitnesspal says for me to loose 2lbs a week without breast feeding i need to eat 1700 calories so how many should i eat to ensure my milk supply doesnt drop - i cant find anyway of adding it too my profile?

Thanks for your help

Estelle