Dance Floor. What size should it be?

We rent portable dance floors throughout the year.  They can be set up indoors on carpeted floors to provide a safe and proper surface to dance on.  They can also be set up a grass surface under a tent.  To do this we first install a plywood sub-floor   on the grass to create an even surface then we install the dance floor on top.  

What size dance floor should you have for your event?  This is how we determine dance floor size.  At most events, never more than 50% of the people in attendance dance at any one time.  Each couple dancing on the dance floor will require approximately 9 sq. ft. to dance.  So, if you have an event with 140 people in attendance, we would calculate that as 70 couples.  Of the 70 couples only half will be on the dance floor at any one time.  Therefore, 35 couples needing 9 sq. ft. of dance floor each.  35 x 9 = 315 sq. ft.  This can be accommodated with a dance floor size of 16' x 20'.