As a general rule you should dial in your espresso using a scale. The first three shots is normally indicating your espresso flow and the appearance of how well your espresso flow/brew.

Your first indicator should be the first drop appearing after activating/starting the water flow (pump/bottom). I recommend the first drop to appear approx. at 6-8 sec on most machines.  in that way your espressoshot will brew as long a 26-31 sec.

If it appears before 4 sec. I will recommend you should grind finer and visa versa if the first drop appears later than/after 8 sec or more.

Dose 20 g dobb shot
Extraction 26-29 sek
Target mass in weight/liguid 38 g.
Extraction rate 20/38= 0,52%

The dose is calculated using a 20g VST porta filter.

Min dose is 19g/ and Max dose 21 g. pr dobb espresso.