The 2:1 method is an approximation of stress distribution in the soil. It assumes the influenced area undera footing cn be defined py lines at the coners of the footing projecting downward on a 2:1 slope. Thus if you have a footing m wide by n long, supporting a load Q, the uniform contact stress at the base of the footing would be
Q/(m*n). At depth z the contact stress would be
Q/((m+z)*(n+z). This is becuse th half width, in say the m dimension would be m/2 +z/2.
