I think you will know when you get there. When I started out I said I would stop at 10 and a half stone, now i'm close to that it turns out i'm still a size 16 in jeans and not really happy as would like to be a 12. Now i'm going for 9s3lb as hopefully it will get me into 12s and it's a nice round loss for me of 7 stone.
Try and focus on how your clothes are fitting etc and that should guide you towards the number you would like to see on the scales. Best of luck! X