You must read and understand the disclaimer
I disagree with the calculation of the factor C given by some commercially available design software and present here an iterative method for calculation of factor C, based on Appendix R. If you plot the figure using this method wou will find that it very closely matches the figure printed in BS5500:1991.
Unfortunately I no longer have access to the current edition of BS5500, and am unable to present the equations in original form at this time. If you can help, please email me.
You may use this fragment freely as you see fit providing you accept the conditions of the disclaimer, though I do insist that you email me if it is incorporated into your software.
Iain W. Bird