I asked the same question about a year ago. The conclusion was that no scale is really accurate except postage scales.

I bought a digital fish scale for about $30 and it read 7 lbs with a known 10 lb weight for example.

It WAS reproduceable, and I guess with weights like 1, 3, 5, 7, and 20 you could plot points and then use it for accurate weighing.

So, whatever scale you get, get something with a known weight about the same weight as your plane and work from there.