You can use the countif function to return all the amount of x and o then work out the percentage...
off the top of my head something like..

=countif(c1:c100, "x")*(100/(countif(c1:c100, "x")+countif(c1:c100, "o")))

percentage=howmanyx*(100/(howmanyx+howmanyy))

I hope this helps.


omin