Writing the Nernst equation for the Ni/Ni2+ electrode would be the first step.
not used to calculating conc. from the Nernst. But, anyway here's what I came up with...
Ni2+(aq) / Ni(s) // Cl-(aq) / AgCl(s) / Ag(s)
I know the E for Ag/AgCl elec. sat. w/ KCL is .197 volts and is used as E (left).
E (cell) = E (right) - E(left)
I am fairly confused on what to do now. Would E (right) be the .8 volts observed? And which form of the Nernst equation do I use? I feel like I am working with not enough information. Any help is very much appreciated.