I work daily with precision analytical balances,I'm a research chemist) that offer the precision and resolution we're looking for here, but those instruments cost thousands to tens of thousands of dollars. To reach a price we're willing to pay, some corners must be cut,vendors won't line up to sell to us below cost). Unfortunately, the instruction manuals accompanying our scales generally aren't very good at spelling out the steps necessary to have then operate to our satisfaction.
For stable zeros it's necessary to warm up for at least a few hours,they're generally left on continuously to avoid drift) and keep the room temperature fairly constant,within a few degrees).
Inexpensive scales such as these are commonly bothered by power line noise and transients, but those van be eliminated by using a filtered power strip; I use and recommend a Tripp-Lite isobar.
Static electricity can easily shift readings by a few grains, so it pays to wipe down all plastic surfaces with an antiststic such as a drier sheet. Don't wear nylon or other static-prone fabrics when working with the scale. A humidifier set to maintain 35% RH will help a lot.
It is necessary to isolate the scale from drafts and vibrations. Laboratory scales with this sort of resolution generally have a housing around the pan with sliding doors for access and vibration isolators in the feet. An air current that's not eve perceptible can easily shift readings at the level of precision we're looking for, but opening and closing a door is a nuisance when you're trying to up the pace. I use a carboard file box,intended for archival document storage) with one end cut out placed over the scale. The side goes down so the box can be removed for filleing with powder, etc. without disturbing the scale. I leave off the top,now facing forward) so I can work my Pact scale/dispenser. With the bench closing the bottom and the box closing three sides and the top, air currents have a hard time getting to the scale.
Vibrations are another problem - you don't want to have heavy equipment operating nearby, or foot traffic on a residential floor. You can make a vibration isolator by putting a heavy plate,Steel, stone, concrete, etc.) on a slab of soft foam, but it may not be effective; really good vibration isolators are expensive.
