That reading is not for chain stretch it's for cam timing. Now that you know what the cam timing is you can use that as a bench mark. Somewhere there is a chart that shows what the cam timing spec is for you cam. If the timing is off you can get an off set woodruf key to correct this.
Also go ahead and check pump timing. late pump timing will give you lousy performance and poor fuel economy.
I don't think you can use that measurement to determine when the chain should be replaced.