If you are a bigger build, your healthy weight is likely to be at the top of your healthy BMI weight range.

You can definitely use body fat to figure it out. Women should generally be carrying between 16-20% body fat to be; healthy. I think you will find that unless you have some serious musecle mass, that once you get there you will be in or near the healthy weight range at any rate.

If it doesn't 'seem' possible or realistic at the moment to focus on such a large amount of wieght, then don't. Loosing just 5-10% of you starting body weight has huge health benefits. So set a smaller goal around there and then once that is achieved set a new goal. I lost 30kg, I done so looking at each 5kg block otherwise it was just too overwhelming. Be aware also, that the closer you get to your healthy weight, the harder it will become to loose the weight - you body becomes alot more efficient at what it does in terms of using energy. Towards the end, I was just aiming for each individual kg.