I have owned the Infuser and switched to the double boiler more than 5 years ago. I'd think that the huge swing you are seeing with your shot duration with one click adjustment is due to the grinder you are using. Never had a manual one BUT you may need to spend more and upgrade to a more capable grinder. Also, with Breville machines, the pressure gauge is very helpful. Just aim to keep the needle in the recommended zone during your extraction and do not try to hit the recommended duration exactly rather give yourself a time window like 25-35 seconds. This is what Breville recommends anyways, there are really good tips in their manual BTW...

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