You will know when to stop. If I drop below 10st I feel very weak on the bike. My "optimum" range is between 10st and 10st 4lbs. I'm 5'9".
We must be rather different builds. I'm 5'8" and if I go below 10st 10lbs I look (and feel) on the verge of anorexia.
Build affects how fit you are for a given weight, as does the speed and reason that weight is lost.
I had pneumonia a very long time ago and my weight plummeted from 11st to about 10st 3lb.
I *knew* I wouldn't feel well and fit until I weighed 10st 10lb and did not.
I'm 10 stone now, still attempting to lose fat and feel fine.
Body measurements and skinfold thickness are possibly more useful than crude weight.
I have got my hips down to 40" (101cm), which is less than I've been for decades; that's still BIG by many standards but mostly reflects the way I am built.
My waist is down to 28"/71cm; that's fairly trim but could lose another 2 inches/5cm.