Last year I lost a plant by accidentally planting a shorter variety in the middle of all the tall ones - it got zero light, didn't grow and didn't flower. I've learned my lesson. The 5/4/3 layout is simply because I have an existing fennel plant and a big rock to factor in! Dahlias seem to love this bed though - it gets sun most of the day, and is at the side of my house so also benefits from residual heat off the wall of the house during summer evenings.
Last year I lost a plant by accidentally planting a shorter variety in the middle of all the tall ones - it got zero light, didn't grow and didn't flower. I've learned my lesson. The 5/4/3 layout is simply because I have an existing fennel plant and a big rock to factor in! Dahlias seem to love this bed though - it gets sun most of the day, and is at the side of my house so also benefits from residual heat off the wall of the house during summer evenings.