How did Busujima Eiji figure that it was about boobs? On the third koma on "As Is" chapter, Nanako wrote,


  Eighty and a five
    Prolly about fifty five
    Eighty plus a two


How did Eiji figure that it was about the president's breast size?
This relates to how clothing manufacturers measure the bust, waist, and hip of women, also known as a woman's "three sizes".  From this, Nanako is basically reading off [what she thinks are] the club president's sizes are.