I lost a massive amount of weight just prior to transition and started putting it back on over the course of HRT and grew boobs. Now I'm losing weight again to get back to where I'm happiest and my boobs aren't shrinking. While not in any way a scientific opinion, I think if you're able to get your fat up a decent amount it will go where your hormones are telling it to and you'll probably get to keep a good deal of it in key places when you take the weight back off. Just eat a lot of peanut butter, avocados, cheese, yogurt, rice, nuts, fruits... calorie laden but healthy-ish foods, get over 2000 calories a day and spend your free time on the Xbox or reading.