How to choose your ideal weight

What is your ideal weight?  It is one of the those perplexing questions that doesn't really have an answer.  It comes down to a few factors..

1. what science says is your healthy weight range
2. what is a maintainable lifestyle for you
3. how you feel about your body
4. your health

Now I will break down that down, using myself as an example.

When I started on my weightloss/fitness journey, I started at 69.9kg - right on the border of 70kg.  Now for a lot of people, this is a fantastic healthy weight, but I am both short and fine boned.  Discounting the second factor, height and weight together give you a great general guide as to where you need to be to reduce health risks associated with weight.  These will allow you to calculate your BMI.  Now, BMI doesn't work for everyone so take note if you may fit into one of the exclusion groups!
At 69.9kg I was also right on the border of obesity.  At this point, I just wanted to be healthy so decided to set my goal at the border of a healthy weight range.. that was to reach 59.9kg.  Now if I use BMI as my only guide, then I get to sit anywhere between 48kg and 60kg.  That is an awfully large range and for some people, the top of the range is perfect, for others the lower end is perfect.. but how do you know which to choose?
http://michellebridges.com.au/tools/bmi-calculator/
The next thing I looked at was 'build' charts, or the 'met life' tables.  These are completly outdated and you need to be careful to an inch to your height, and if you are really really short, or really really tall.. they are way out!  I use my wrist as an indication of build.. I can wrap my finger and thumb around my wrist and have them overlap which means small build (touching is medium, not touching is large build)  - this gives me 108-121pounds or 49-55kg.  It fits with the BMI and gives me a second aim.. to be under 55kg.
http://www.halls.md/ideal-weight/met.htm
Lastly, I look at my waist measurement but for others it maybe one of the first things -depends on your build.  I am unlucky to be an apple - I carry nearly all my weight in my torso (and not the boobs!).  My waist measured over 90cm before I started the 12wbt, over 1m when I was almost 70kg.  Now it is sitting just under 88cm which is fantastic... that was my first 'measurement' goal.  At 88+cm you are at a much higher risk of disease due to visceral fat around your vital organs, the next milestone is 80cm - once I get below that I will be in the 'normal' risk area.  This the vital information that prompted an entire government campaign that you probably already know.. the measure up program! 
http://www.measureup.gov.au/internet/abhi/publishing.nsf/content/home
The next measure you may want to look at is body fat.. I am at about 28% body fat which is healthy but not ideal. This is more for athletes and those building body mass however if you are dropping below 20% bodyfat just due general health and fitness then you need to think about the risks you are taking with your health.  To find out your bodyfat percent you can get a professional scan done (dexascan), or see your doctor or local gym to get a caliper test done.  I don't recommend body fat % scales as they seem wildly inaccurate unless you are spending $250+ on them.

Now, that is the first factor out of the way - the rest is really up to you.  You need to take into account the effort required to get to and maintain your 'ideal' weight.  If you have to regiment your diet and exercise to maintain then chances are it is not maintainable long term.  Unless you are a professional and are working toward a particular event, then want to be able to eat without counting calories and be able to enjoy your food and the occasional glass of wine.  Comfortable and healthy and easy - that will all determine were your 'ideal' weight lies - one that is maintainable long term for you and your lifestyle!

Next thing.. we all want to look good but hey, my 'good' might not be yours!  I want to be able to see some muscle so want to get as low as I can, but others are quite happy to maintain at a level where they have some softness and curves to their body (we all know that the boobs go first!).   This is such a personal thing and although not health related, it is still important!

Lastly - we want to be healthy!  Unless we are training for a specific competition, we need to have body fat in store for sickness, and to maintain a healthy balance in our bodies!  Too little body fat and we lose more then weight - our periods, hormones, growth, moods, hair, and all of our health will start to go.  If you are losing weight and starting to get sick more often, feel lethargic and run down or feeling any kind of unwell, you need to stop and think about your health.  Sometimes a rest is all we need, sometimes we need to listen to our bodies and realise our limits.  I was 43kg at one point in my adult life.  I developed a mental illness that I will never fully 'recover' from.  It will always haunt me.  Cause or effect?  I don't know, but I do know that maintaining a healthy body is essential for mental health!