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 8-10 sec. In that way your espressoshot will brew as long a 26-31 sec. if it appears before 6 sec. I will recommend you should grind finer and vise versa if the first drop appears too late/after 11 sec or more.
This extraction will give you a more ristrettoét espresso and more intens.

Dose 21 g dobb shot
Extraction time 27-29 sek
Liguid mass 38 g.
Extraction rate  21/38= 0,55%

The dose is calculated using a 20g porta filter min. dose 19g/ max. dose 21 g. pr double espresso.