Now that Genghis has made comment, the following may be useful also.
Under the old Australian ANO 101.22 rules, the local AFMs for the C172 have a graphical presentation which would be very useful - these were conservative, but not excessively so. Similarly the mathematical model upon which that style of chart (commonly known as the P-chart) was based are available via a (then) DCA published light aircraft how-to-do-it manual. The original document was a very simple analysis. Some years later a much more complex analysis was adopted by CASA - the earlier one would be far more appropriate for your buddy's requirements.
My copy is back in my office in Melbourne and I am not able, conveniently, to gain access to it from where I am working at present. You might consider approaching the Australian CASA to see if you can obtain a copy. Alternatively, when I am next in Oz, if I remember to do so, I can look the document up and send you the specific equation which was used.
[ 06 October 2001: Message edited by: john_tullamarine ]