I know this might sound odd and it certainly won't give you an exact figure but what about getting hold of a second set of scales, putting it next to the first scale and putting one foot on each scale, try to maintain an even balance between the scales and add the totals together .... this might give you an idea of where you are up too but don't count on the final number being exactly right .....

Another option is going to a post office or a shop that sells scales, ask them nicely if you can use them - most places I've asked said yes and have been really nice about it .....

What about your doctor, does s/he have a scale that goes higher than your own ..... maybe you can weighin their on a monthly basis .....

Just a few suggestions but honestly I would suggest the following

Weigh yourself infrequently (atleast 2 week intervals) its not worth becomming a slave to the numbers.

Measurements are far more important as its already been said here, they tell the true story!

HTH

Fiona