Hi Jo,
starting at 20 stone 5lb I should lose half my body weight too. According to my BMI I should be 9 and a half, however I will not go by that I will go by what makes me feel good. I am aiming for 12 stone, with the proviso I may come off this at 14 if I feel it was enough and felt able to carry on with WW or some other diet. I need to get there first before I decide, as it will be a visual and feeling thing for me, not having been there before, well not since I was a teenager - too long ago to remember.
I set different goals for myself, 18 stone - just passed and the next one is 16. I know now they are doable and they are not so far into the future that I can't visualise them. I would advise you to pick a realistic goal and aim for it, it could be a clothes size, a trousers, shirt, weight, inch loss, whatever but it may keep you focused.
I want to wish you all the best with this diet, it is wonderful and once you get over the first week and into ketosis you will be flying. Can't wait for your first weigh in. Best of luck.
Doirin