As flat as possible? Isn't the idea that the water should have contact with a definite amount of coffee before reaching the cup, like those "V60", etc.?
Discussion
Correct. That's why you have to pour around the edges too.
If you only pour on the center, you're pushing the coffee outside, creating a "crater", and eventually you'll be basically pouring water directly through the filter.
I try to put the ground coffee in the filter making a mound too, to have the thickest shape possible in the center. That helps a lot too.