When I learned espresso, the golden parameters was brew ratio 1:2 in 22-30 seconds. So, if you got 18 grams of coffee grind going in, you should go for 36 grams of brewed coffee in the cup.
This fits dark roasted coffee well. But with a light roast the espresso shot is very acidic and dry.
There have been several work-arounds to make it better. Strech the roast profile. Brew with longer preinfusion and at a lower pressure.
Here is my work-around …
Search your coffee by tasting drops during the espresso shot
Let an espresso shot run really long while you taste with a spoon to follow what taste is extracted along the way. Provided that you got a suited grind size and don’t have channeling – the development in the taste is:
Very sour and sharp at first
Then sweet and nice aroma
With a dark roast this is released fairly quickly: with around double the amount water than coffee – the 1:2 brewratio.
But light roasts are denser and got less solubles than dark roasts. So the taste is released slower. This is why longer preinfusion makes sense. But also running a longer shot, like 1:3.
When I brew a light roast espresso, I always explore the coffee on hand by tasting while the shot is running.
Using 17 grams coffee in the portafilter, I skip the first 15-20 grams because they are always very acidic.
Then I taste. Only a drop at the time (or it will get too hot).
At a good grind size the good taste will start around 30 grams in the cup – and keep on until 50 to 70 grams. When the taste starts to fade or get bitter, I stop the shot.
If the shot was stopped at brewratio 1:2 (here: 34 grams) it would have been very acidic. Most of the sweetness and big aromas would have stayed back in the portafilter.
The best way to know how what brew ratio suits a given coffee, is to taste during the shot.
See my video https://www.youtube.com/watch?v=VCovMAG2LXM
Read more about the differences in light and dark roasted coffee.