Calculate Geo Mean in Crystal report; thread767-1586227

[visit2me]

I read previous thread dated back on Jan 2010. by LisaRR (Geometric Mean)
link to thread:

http://www.tek-tips.com/viewthread.cfm?qid=1586227

or thread767-1586227

Thanks everyone for their input on LisaRR thread. It works great for me too for each month calculation. however, I am in a situation that I need to calculate and display based on previous two months geo mean for current date and keep display records for last 12 months lines.
For example:
if i run my report for dec 2011, my report should displays 12 lines, one line for each 15 previous months. And display geo mean on each line based on previous 2 months. i have a group in my report on Month

Any help will be greatly appreciated,

Visit2me,