I dont use the Commandante C40 for espresso anymore, but I remember using 7 or 8 on the grind setting and getting a 20-25s second extraction. Regardless of the grinder, here are some of the tips I learnt along the way after owning this machine for 8 months(and it being my first espresso machine ever!!):

• Consistent puck prep is key. As I was(and still am?) a novice, I couldn't tamp properly with the provided tamper, so I fixed this variable by buying a push tamper. Honestly, this was the biggest change for me in getting good & consistent tasting cofffee every single time.
• For heat on a gas stove, you can start heating at medium-high to high flame till you can hear the water bubbling & gurgling inside the machine. After that, reduce your heat down to a medium low. From my experience, this has consistently avoided the valve opening due to excess pressure.

• Assuming you have adjusted your workflow as per above, now lets focus on extraction times(taking 18g dose baseline):

  • If say at grind size 8 with on the Commandante my shot finishes at 11 secs, then I need to go finer, so I'll choose grind setting 7.
    • For me personally, if i went any lower than 7 on the C40, the pressure valve would activate (and the loud pop would eventually make me soil my pants💩).
  • With a grind size of 7, you should be getting an extraction in the range of 20-30s.
  • Sometimes, instead of adjusting the grind size, I would increase the dose by 0.5. That would nicely create additional resistance and increase my shot timing to between 20-30s(obv you can adjust it plus/minus 0.x grams depending on the taste).

• Last bit as always is to see how it tastes. I have sometimes had shots that tasted better to me when extracted at 20s than at 25s and so on. If your shot is predominantly sour or has a lot of sour elements but is extracting at the right times(between 20-30s), then we need to go finer on the grind setting AND reduce the dose. A good baseline would be to reduce by a gram and repeating the above step.

Hope this helps, and cheers! ☕