- The value of the variable must say something about your performance with regard to the relevant property/task
- It must be easy to measure/record
- You should be able of adjusting it by performing relevant actions
- Is its ideal value sensitive to external happenings outside of your control?
My suggestion is to pick a set of candidates for KPI's and then analyze them for their potential as good indicators of performance. To each KPI we put up a set of relevant actions, such that we can push the KPI in either direction as needed. Then, for each KPI candidate evaluate whether it has potential to tell you the relevant things about your performance, also when unforeseen things happen. Select the one that has the most positive indicators and try to identify its best value. Then, for a while try to "control" this variable at its ideal value by using your designated actions. Hopefully, this has led you to focus on the right things. This was how I came up with counting co-workers coming in to discuss work, and those who were only in for a chat as a good KPI for seriousness.