I have to let my Sherline "settle" to get an supposedly accurate reading. Once I put all of the weight on the scale, I add some of my body weight to the tongue a couple of times, and move the tongue gently from side to side. If I don't go through those motions, the scale will read lighter than the actual weight.
Once I let that puppy settle, it reads pretty close to the same readings I get from the CAT scale......within about 20 lbs. or so, or as close as I can guesstimate from the increments on the scale.
Another thing to consider is that the Sherline is most accurate about the center of the dial. If you expect a 500 lb. tongue weight, don't get the 2000 or 5000 lb. model.......get the 1000 lb. unit. If you are expecting a 800 lb. tongue weight, then the 2000 lb. model might be best. I have a 700 lb. weight, and use a 1000 lb. scale......that is in the middle 40% range of the scale. A 2000 lb. scale may have worked better as well as that would have been in the middle 30% of the scale, but the increments would have been closer together.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263