This article develops simple and easy-to-use approximation formulae for the length of a Chinese Postman Problem (CPP) optimal tour on directed and undirected strongly connected planar graphs as a function of the number of nodes and the number of arcs for graphs whose nodes are randomly distributed on a unit square area. These approximations, obtained from a multi-linear regression analysis, allow to easily forecast the length of a CPP optimal tour for various practical combinations of number of arcs and nodes ranging, from 10 to 300 nodes and 15 to 900 arcs.