I think 28lbs in 7 weeks is a little ambitious - that's 4lb average loss a week! Why not aim for a stone in that time, 2lb is much more achievable and you won't be putting yourself under a huge amount of pressure - any more than that, and it is possible, would be a bonus xx
You can aim for a stone i think as if you keep to plan and are active 2lbs a week with consistency isnt to big a thing to aim for. Depends how much you have left to loose. Thing is if you go for 14lbs and loose even more bonus