Hmmm... Good question.
I'm thinking some kind of visual thing?
Maybe a chart with a color-coded system, say with frowny faces and a corresponding color, so blue is a little bit mad, purple is medium mad and red is "you're in big trouble, Missy" -or whatever system/ colors work. Then you can point at the chart and let her know where you stand.
I was initially thinking a traffic light system, but I don't think that has enough colors to cover the range. Maybe that will work to start the concept, and you can build on it?
Make sure you include a "not angry" in there because I know my kids will often think I am mad when I am not (or often think I am mad at *them* when I am mad at something else).
That's all I've got. Sorry.
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Nathan does this too.
You just described my children LR.