Do you aim to keep the brew time around 30sec regardless of shot size? If so, how do you go about that? Do you grind coarser? Tamp less hard?
If i keep my grind settings the same and tamp with (what feels like) the same force, my extraction time takes a lot longer when pulling a double shot (i use 16g) compared to single shot (8g), and using the same ratio.
The “perfect” logic is 30seconds, at 93c, of 18g coffee, gives 30g liquid.
But you need to vary the roast, time/temp, grind size and grind amount to all give your perfect coffee.
I always tell people you want to see the oils in the basket after you have pulled the shot as a good sign you are doing it right.