To my knowledge there are 3 types of scales, the least expensive is a load cell. The most common in better scales is called magnetic force reconstruction and the third type uses a tuning fork system
Load cells are prone to overloading and are known for poor accuracy.
The FX 120i is magnetic force reconstruction which is generally quite good but it is vulnerable to a variety of influences such as voltage fluctuation and even interference from radio waves such as a telephone.
To get best results from a magnetic force reconstruction balance you need to use some sort of voltage stabilizer, like you might use for a computer and keep it away from phones.
Any scale can drift in response to changes in barometric pressure like what might happen when the air conditioner or furnace come on as well as a dryer or even gas water heater.
The best scale I know of is made by Vibra and I have one as well as a few of my friends, but it does not connect to any sort of auto trickler that I know of. The Vibra is a tuning fork balance and is not affected by voltage changes, it also does not need to warm up like other types.
The more sensitive the scale, the more it will be affected by air pressure, so its best to close windows and turn off major appliances when in use.
I would suggest that you make some sort of validation weight that is close to the pan weight plus your target load. Consider a length of clothes hanger wire. Cut it to a length that equals the pan weight plus the powder charge weight. Use that to confirm the scale has not deviated from this known base line value.