There are other techniques, for instance: comparing different perimeters.

My favorite method is measuring abdominal circumference. It’s very simple method, requires only a measuring strip and it can tell you in what direction you are heading (deltas). Note: measure always in the mornings before breakfast and do it always in the same way (regular standing, without deep inhale).

Here is a calculator that contains several different techniques together: http://home.fuse.net/clymer/bmi