The Geminis are are about as good as you'll get not paying $350 or so. Make sure to do the measurement with a 10g weight under the cup; they're most accurate in the middle of their measurement range. Even then, their resolution is probably an order of magnitude off their stated value; the calibration weights needed for that kind of accuracy would cost more than the scale itself.
Stuff to do to maximize your accuracy:
1) Put the scale on a very stable hard surface.
2) Measure the weight using the middle of the scale range, i.e. put a 10g weight on along with the sample to be measured
3) Exercise the scale a bit before using it. This means placing a 10g weight on it. Hit "tare" wait a minute. Take the weight off, do the same thing. Then do it with 20g.
4) Keep the same temperature between readings. The scale uses a strain gauge for measurement and the materials can be affected by heat.
5) Pick up your calibration weights and samples using tweezers to avoid depositing oils or other stuff on the surface that can throw off the reading.
I don't think it's completely necessary to have the sample directly in the center because of the strain gauge used.
Finally, the very worst part about the scale is that it has an "auto-off" that can't be turned off. You'll be adding stuff to the scale to get your desired amount and then it'll turn off. Who really needs to save batteries on this stupid thing? It's not powering a light bulb or heating up or anything. The auto-off "feature" (BUG) alone is why I've been searching for some replacement, or perhaps going to hack this thing to do my own measurements. Totally pisses me off.
