What’s the best way to determine how to rank the severity in 8 wastes?

This is not a sophisticated tool so those levels are up to the user. If you are conducting the assessment with others, you can determine operational definitions together based on the process. Some people like to conduct this assessment with other process participants so they can agree together on how to rank the different wastes. That’s a popular way to use the tool since it leads to great discussions. Depending on the situation, you can also include the Champion in the assessment if they have a good sense of the waste in the process. Both are good options.