Got a better idea for you DC
Tell her that she is looking to thin and that you are worried about her
That should start a discusion/argument about how much she weighs
Easy to manover her into proving she weighs **stone as you just say that she looks too thin to weigh that much
She will probably get on a set of scales just to prove you wrong
Voila you have her exact weight and probably a Girlfriend that thinks you are the perfect man
Only one way this could go wrong mate
If she jumps on the scales and is 2 stone heavier than she thought
Then your in the **** anyway so don't blame me
Jason