Counterion charge difference and membrane voltage ($Q_{\mathrm{in}}-Q_{\mathrm{out}}=2Q_{\mathrm{in}}$)

For an ideal two-plate capacitor with opposite charges, a stated inside-minus-outside charge difference is twice the inner plate charge. The voltage is inner plate charge divided by capacitance. Confusing a plate charge with the difference of plate charges creates a factor-two error in inferred membrane voltage.