Dialing in a Sage Barista Pro woes

I have got a refurbished Sage Barista Pro and it's a great machine, but I'm having some problems dialing it in.

I have some medium (I think - bag doesn't say) beans that I'm usinga dose of 18g of per go round. Last night I set the machine to 8 grind size and pulled a shot. I managed to get 36g of espresso out in around 29s, just outside of the ideal. I changed the grind this morning to 7 so it was a teensy bit finer, but now I'm getting 36g in around 20 seconds. Moved it back to 8 and a similar timing occurred.

I've used up nearly 500g of beans and not had a single coffee yet - am I doing something wrong here? I have been moving the grind size down gradually but it doesn't seem to be getting closer to the ideal yield time.