Help with Counter Weight formula

Hi Y'all! Ok I have a problem and I hope one of you guys out there might be able to help me: I am well over the weight range of a normal Doctor's scale and buying one that would accomodate me would be cost prohibitive at this time. I remember years ago when I went to TOPS the lady there who would weigh me each week would use her keyring as a "counter weight". She used some formula that seemed simple at the time but I can't remember it. Does anybody know what I'm talking about? If anybody knows this formula please pass it along to me! Thanks and hugz! Jonne

check out ebay. I bought a scale that went up to 550 lbs on Ebay for about 40 bucks....