If your bathroom scale systematically estimates your weight to be 5 lbs too heavy, the subtraction method
[(human + bike) - (human)] = bike
gets rid of the systematic error. Since bathroom scales tend to work best between 150 and 200 lbs, this is a perfectly reasonable way to obtain the approximate weight.
So, if the OP of this necro-post still happens to be around, save your money on the fish scale. Those things notoriously exaggerate. ;-)