What variables should I look to eliminate in setting up an electronic scale (in this case the Gempro 250)?

I don't know what happened but recently I started to get readings (after calibration) that were in some cases only 90 percent of the true mass.

I don't have any florescent lights around the reloading bench.

The only calibration weight I have came with the press (20 gram).

I use the A/C power adapter to the press.

This naturally concerns me as I got LAZY and didn't verify a new load with my beam scale.  Instead of 22 gr of powder I was throwing 24.6gr.  The added recoil is what made me go .

In other words, pretty much a textbook example of what not to do.

Leave it plugged in 24/7 and just turn off the display. Needs time to warm up before calibrating. By leaving it on it's always warmed up. I suggest buying a scale check weight set. I don't calibrate very often or ever. I always use my check weights to verify before loading and my GemPro and Pact electronics have never needed calibration except when new. I used to calibrate every loading session until someone recommended to leave them plugged in, tare then verify the weight given is accurate. Great advice.  If nothing helps, send it back. I would definitely buy a scale check weight set. I have Lyman's. I believe GemPro's have a lifetime warranty?  Wouldn't use electronics or balance beam without check weights.

I use check weights and (almost) never calibrate my gem pro.  Check weights are one of those things you just need to buy.

Ah, a check weight set.

Ordered!

Thanks!

I didn't think of leaving it plugged in, either.

Once my 250 has been on for over 24 hours, it seems to settle quicker.

I strongly concur with the notion that a check weight set is important.  I DO calibrate my scale, but I use check weights to verify that it tracks linearly.  So far, not an issue.