I don't use the cover on my Chargemaster except when I have removed the powder and powered it down.
I always calibrate the scale every time I turn it on.
If you don't you can't be sure what you're actually measuring.
I have found that the scale must be level if I expect the measures to be repeatable.
A powder type will occasionally cause a over run on the last bit of powder if the powder is relatively bulky. The fine grain powders meter very well and don't exhibit that much.
Humidity sometimes causes the powder grains to clump up a bit and may cause a 0.1 grain variation.
Vibration can result in some very strange results so the scale needs to be on a very stable surface.
I always let the scale settle after it "beeps".
The beep only means that the scale reached or passed the set limit.
Then the scale displays the count for that dispensed load and then shows the final result (actually what the dispensed load weighs).
If there was any clumping or any vibration while the last bit was dropped, you might be a 0.1 grain or more heavy.
If the scale was shaken during the measurement process, the Chargemaster can stop and you might find the load is light.