How much weight do I need to lose?

I am 5'4" and 100 pounds. I have a small frame so I don't look skinny. According to one BMI calculator it says I am 17.2 and another says I am 20 (the one that is more detailed with waist, neck, and wrist measurements). My doctor said that I am not at all underweight because of my body structure and said only anything below 15 BMI is underweight (dont criticize, i have an unusual body and she knows what shes talking about). How can I get down to the 15-15.5 BMI percentile? I promise you it isnt unhealthy, it is just my body type. No mean comments. Also, I regularly exercise, and how many calories should I eat a day to lose this much by August 15? THanks!