Both the hand held device and the online calculator wouldn't be too accurate. I have a measuring device that cost $2000, it asks for a large range of questions to be answered and even though it uses all that info I still doubt the results with some clients. The calculation you use would we an estimate based on a study so it's accuracy would depend on how the research was done (ie, were the subjects overweight or atheletes, age, gender, how many wer tested, etc).
I know it's good to have an idea on where you're at so you can measure your progress but I'd stick to waist measurements. It's simple to do and very reliable (ie, if it's getting smaller, you're making progess!)