Welcome HealthyH
You need to take note of your own body and feelings, but I would suggest that you DON’T add in extra calories on the days you exercise.
You are changing your hormonal response whilst on the BSD to burn fat for energy and not quick release carbs. So if you exercise, but still stick to the 800 (average, you don’t have to hit it on the nose each day), you will lose that weight quicker and train your body to get the energy from those fat cells.
As you start, you may have a bit of ‘carb flu’ which you may have read about, and this can (but not always) lead to feeling a bit tired to start off with. But this should soon pass and strangely even on 800 calories (which some people think is a ridiculous level, I know) you will feel full of vim and vigour.
I did the whole BSD, exercising the while, on the 800 calories and can say that it was absolutely fine.
But of course, every individual is….er…..individual, so if you feel weak and weebly, then you may need to tweak your calories.
The main thing is, if you do this, to try and keep your carbs (the good veggie ones) at 50g or less per day.
And just to add, absolutely no issues with your OH and pre-diabetes. My OH was Type 2 for a long time until I bought this book (I needed to lose some weight and had no idea how to do it, despite success previously) and told him that he might be able to get off Metformin if he followed the instructions. He saw me for a few weeks, joined in, and now is Type 2 ‘in remission’ on zero diabetes pills, and feels wonderful. We are loving this way of eating. 🙂