First of all you should not be adding calories if you want the full effect of the BSD. Secondly, let your body tell you if you are doing too much. I swim twice daily for around 40 mins each time but unfortunately I cannot do it on this diet. You do not say if you are diabetic. I am, so it is important that I do not go hypo, if you are not diabetic you might just feel a bit weak or more tired. For the first 2 weeks I could only do a little exercise so was swimming for 20 mins each time. I now have the timing down to a fine art. I don’t exercise when I have not eaten for a few hours. I have my meal, rest for half an hour then exercise, although I must say I have not managed to get back to 40 minute sessions. I also save some of my calories so I can have a snack within the 800 so, for example, today I had a meal that should have included pistachio nuts but I left them to one side to eat later.
If you are doing this just to lose weight then maybe there is a compromise you can make which allows to eat a little more in order to continue with the exercise. There is advise about exercise on the FAQ’s. I know people here like the MyFitnessPal but any machine bought for the mass market is bound to be inaccurate.