Bishop & Morgenstern method for slope stability

I'm pretty familiar with the Bishop and Morgenstern method for the calculation of the factor of safety for simple slopes. However, I have a problem that deals with a 45 degree angle (beta) between the horizontal and the slope surface and the tables developed for this analysis by Bishop and Morgenstern do not list m' and n' values for a slope of 1H:1V (45 degrees) in most geotechnical engineering textbooks. In this particular case I have, C'/(unit wt.)(H)= 0.116. Is there some kind of rule for this type of situation when you have a 1H:1V slope to solve for the factor of safety when the books do not list m' and n' values for that particular slope? The final equation is:
Fs = m' - n'(Ru) (after m' and n' have been found) where Ru = pore water pressur parameter (in this case, 0.5). Can anyone help me on this?

Thanks