Method II


  Construction of the grid :  
  1. Draw the diagonals to create M (figure 1), draw a line parallel to AB through M (figure 2). Measure the intercepts a and b and divide AB accordingly to create F. Draw a line e through M and F.  
  2. Do correspondingly for AC to create g (figure 3).  
  3. The lines e and g present the base of the grid (figure 4) which can now be completed as shown in figure 5 and 6.