〖WTBX〗P〖WTBZ〗B  is given as a function value of lightning protection level (LPL) in the national standard GB/T217142-2008, without considering the sizes of buildings, which is not precise. The model of the struck is established by using electricalgeometry theory. The relationship between risk and building characters is provided. The method related to the heights of buildings and lightning arresters for calculating 〖WTBX〗P〖WTBZ〗B  is given. Compared with the old method, this method is used to calculate the 〖WTBX〗P〖WTBZ〗B value of physical buildings. The result shows that 〖WTBX〗P〖WTBZ〗B calculated with the new methods is generally larger than that with the old one; the practical significance of this method is demonstrated as well.