Putting "Leadership is an art" on my reading list, thanks :-)
About measuring your impact, or productivity, as a manager: I wrote Recalibrate Your Productivity Sensors some time ago, maybe you find it helpful. It talks about how, as a manager, most of the metrics of productivity you had as a developer just go away, and you have to find new ones. I think it arrives at a similar conclusion as you do: Make a difference to people and the organization. Unblock, teach, listen, encourage, give feedback, bounce ideas back and forth, etc.
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