I struggled with exactly this question for almost my entire weight loss. I imagined that I wanted to be somewhere between 10 and 10 1/2 stone, a size 12 - but had no real idea if that would be achievable or realistic for me. So, in my head, my target was 10st 4lb, to allow me to be 10st1-10st7 target range.
But I remember being about 10st 11 for weeks and weeks and weeks - it was clear that my weight was staying the same, but my body was changing. I was now comfortably in size 12 jeans, and was getting irritated that I couldn't seem to get past the 10st11 barrier. I tried everything - I was being 100% already, but I used SE and EE - any plan to get it shifted.
What was becoming clear though was whilst I didn't know when to call target, I knew I wasn't there yet, and I had to keep at it for a bit longer...
Then, one morning a few weeks later, I woke up and caught myself in the mirror and liked what I saw. I was the shape and size I wanted to be. My decision was made, I would do 100% for the next couple of days and call target regardless of what weight I was.
I told the weigh in girl I was calling target before I stepped on the scales (which she thought was rather odd), and it showed me at 10st 9, a loss of 1st 13.5lb. My Consultant has never understood why I called target that night (nor it must be said has she ever asked), but always jokes that it was a really odd weight to call target on (surely another 1/2lb would have been neater!)
So how did I know what was right for me? - For the first time in my adult life I was 100% happy with what I saw. The numbers stopped mattering, and that made my decision for me.