Alan, if you want to explore this subject further, there is a lot of information on the web.
One site which I found helpful in establishing the best proportions of carbs, fat and protein to support the BSD is below.
I believe Michael Mosley set out the BSD to be as simple as possible, so that its users could do the diet without too many investigations/calculations themselves.
However, many people here, as well as my OH and I, have established that planning the 800 calories around 50g or less carbs per day works very well at both losing weight and visceral fat, and helping with issues such as high blood sugar.
Here’s the site:
http://keto-calculator.ankerl.com
If you work out your potential proportions from the site, using the carb levels and calories per day (800 on the BSD)you wish to set, it will calculate your recommended protein and fat levels so that you can enter them into an app such as the one recommended by Christine or MyFitnessPal, which is the one I use – there are a number – and set your targets.
Then entering your food into the app keeps you on track as it shows the nutritional content of what you are eating, and you can monitor your progress, plan your meals, establish how many calories/carbs there are in everything.
I hope that helps.