How do you calculate a half cell potential Copper(II) and Copper Sulfate? I have to complete an experiment for school where I have copper and zinc electrodes placed into an electrolyte of copper sulfate and I have measured the potential across it at varying temperatures. For example at $\pu{60°C}$ ($\pu{333K}$) I measured the potential to be $\pu{0.880 volts}$ and at $\pu{75°C}$ ($\pu{348K}$) I measured the potential to be $\pu{0.780 volts}$.

Is there a way I can prove this result? I tried using the Nernst Equation, but my reaction quotient is equal to 1, which disproves my finding. I found out about activity coefficients, but I do not know how to apply it or the Nernst Equation to half cell potentials.

Any advice or help is greatly appreciated!
Here is a figure of the famed Daniell cell:



Note the spelling of the name: it was invented in 1836 by John Frederic Daniell. At $\pu{25 ^\circ C}$, and with $n = \pu{2 equivalents/mole}$, as correct here, $(\ln 10)\frac{RT}{nF} = \pu{0.02958 V} = \pu{29.58 mV}$.