I use BMI, but go with the range. So for my height (170cm) the healthy "range" is 58-72 kg. I set the upper end of the range as an initial goal weight, and then once I was there, set lower goals. I find that my body sort of "settles" around 68-69, so I think that's what's healthy for me.

Congrats on your 18kg - that is awesome!