It depends on your frame size and muscular structure as well though.
Frame sizes Your frame size measured as measured by the width of your elbow or wrist is correlated to your bone mass and muscle mass. Women generally have a smaller frame size then men for any given height.
Quick Measurment Method
The simplest way to measure your frame size is to wrap your thumb and your pointer finger around the smallest part of your wrist. If your fingers don't touch, it means that you have a large frame. If your fingers are just touching that means you have a medium size frame If your fingers overlap that means you have a small frame. This method of measuring your frame size is not as accurate as the elbow measurment method and should only be used as an approximation. Elbow Measurment Method
A more accurate way of measuring frame size is to measure your elbow breadth against your height. To measure your elbow breadth, bend your forearm up at a 90 degree angle, with your hand open, turn your palm so that it's facing your body. Next take your pointer finger and your thumb of your other hand and place them on opposite sides of your elbow at the elbow joint. Then measure the distance between your fingers with a ruler. The following is data from Metropolitan Life on the medium frame sizes of men and women. If you are below the medium you have a small frame size, if you are above the medium you have a large frame size. Men (Medium Frame Size) HeightElbow Breadth5'2"-5'3"21/2"-27/8"5'4"-5'7"25/8"-27/8"5'8"-5'11"23/4"-3"6'0"-6'3"23/4"-31/8"6'4"27/8"-31/4"Women (Medium Frame Size) HeightElbow Breadth4'10"-4'11"21/4"-21/2"5'0"-5'3"21/4"-21/2"5'4"-5'7"23/8"-25/8"5'8"-5'11"23/8"-25/8"6'0"21/2"-23/4"
According to BMI calculations (which you can take or leave) you should be anywhere from 115 pounds (8 stonne) to 155 pounds (11 1/2 stonne).
Like Yambabe said, it depends on your frame. I'm the same height as you and I don't think I could ever be 115 pounds. People never believe the weight I am, they always guess 20-30 pounds less, so I think I'd be on the higher end of the BMI chart.
Personally, I'd say that it depends... I'm a fan on just looking into the mirror. Does your stomach stick out? Do you have a waist? These indicate how much more to lose. I don't believe that people should use a formula for 'ideal' weight unless it has about 5-10 variables (BMI has 2).
I'd say get down to the higher end of your ideal weight range (in your case 155lbs) then see how you feel from there. That's what I'm doing. Although, my Grandma (who is deadly honest about weight *sigh*) told me to stand up the other day and said "ten and a half stone." So that'll probably be my ideal goal.
Midrange of your BMI should be about right, but as has been said before, it all depends on your frame. I am just coming into the healthy bmi range now, at 5'10, and am still a 14-16! Not what I want at all, I want to be a svelte 12-14... wonder if that means I have to go all the way to the bottom of the range. I can't imagine being that small though....
Is there anything wrong with setting a goal that is to the lower end of your healthy BMI?
I am getting towards 9 stone now and am 5 foot 6. While I am getting happier and happier with every pound I would still like to go maybe that extra half stone or stone further. Is it wrong to aim of a BMI of 20 or slightly under?
No, nothing wrong with that at all.. as I said it all depends on your frame and your build. I think I would look emaciated towards the lower end of my bmi range, and I want to keep a few curves! I am particularly fond of my boobs! lol... but, I think it is different if yu are shorter/a smaller frame. Just go with what feels right for you.