Standard electrode potentials give us reduction potentials. So for a proper cell diagram, you take the more negative (or less positive) one—that's your anode, where oxidation happens. The cell EMF is simply E°(cathode) minus E°(anode), always a positive value for a spontaneous cell.
I learned this the hard way, sketching a Daniell cell for a column piece, while watching a monsoon drip hit a tin basin in our courtyard. The sign of the anode, like the sign of a leak, defines the direction of the flow.
You have to be meticulous with the signs, or the whole calculation, like my father's repair plans for this old roof, just won't hold up.
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