I just played this game today, and I think you have it. Home team was level 7 and had a 43K stadium, 40 K fans and 5% bonus.
According to the table of prices matrix, a L7 stadium is 43K. And according to my calculations, a L6 stadium (31K) with the large seat upgrade (43K-31K = 12K x .75 = 9K) should have 40K seats (31K + 9K) .
Good job