I don't treat this as a science project. I don't need an ultra-precise scale or calibration procedures. I want something that will give me an idea if their weight is gaining or losing. I want something that is very portable and reasonably accurate. I want to be able to check a lot of hives in a short amount of time. I use it in the fall as I open feed. It points to the hives that are not taking the syrup, so I'll put a top feeder on. I have a target weight I use to get them thru the winter. I can tell in the winter if their stores are getting dangerously lite and I can take action to help them out. I also like to watch the explosive Flow. In this part on NC, the Flow lasts about a month and thats about all we get for the year. Everything after the flow belongs to the girls. As you can see on the graph the girls bring it in fast and furious. Then it just stops. The graph's last entry was June 6 and I have since extracted. They are consuming now and the weights are slowly going down I'm sure. I will start weighing again this weekend.
As to your question about discounting for adding or removing supers...no I don't. I have dedicated pages in Excel which is my log...a separate page for each hive or Nuc. I note the date I add or remove boxes, so it will relate to the graph. I tried discounting but it became too much trouble.
I encourage everyone to weigh their hives. Its fun, interesting, informative and when your fellow berks find out you're weighing, they'll be calling you to get an idea of how their hives are doing.