Why your metric defines progress

Let's say you have an objective to improve customer happiness. A key result could then be to increase NPS from 30 to 50. In this case, NPS is your metric. And you'd have to look at your current NPS score to see what the correct value is for your key result. There are lots of things that you can do to increase NPS. You could squash bugs in your product, hire extra support agent to bring down your response time, build highly requested features, but doing these things won't automatically mean NPS will go up. NPS will only go up if more of your users give you a high NPS score and thus become promoters. In other words, only the current value for your metric can and will define progress for that key result.