I don't believe that the feng shui is graded by how many pieces of furniture you have, just that the available spaces are the correct color. Keep in mind that when your room is completely upgraded, you have more area that you can use. I found that the watermelon table put in the lower right corner counted for red and green. I did a fruit themed room and by pacing the furniture in the correct places got a perfect score.
GG loves ACNH!