In an electrochemical reaction of lead, at standard temperature, if $\mathrm{E}_{\left(\mathrm{Pb}^{2+} / \mathrm{Pb}\right)}^0=\mathrm{m}$ Volt and $\mathrm{E}_{\left(\mathrm{Pb}^{4+} / \mathrm{Pb}\right)}^0=\mathrm{n}$ Volt, then the value of $...

$\mathrm{FeO}_4^{2-} \xrightarrow{+2.2 \mathrm{~V}} \mathrm{Fe}^{3+} \xrightarrow{+0.70} \mathrm{Fe}^{2+} \xrightarrow{-0.45} \mathrm{Fe}^0$
$\mathrm{E}_{\mathrm{FeO}_4^{2-} / \mathrm{Fe}^{2+}}^\theta$ is $\mathrm{x} \times 10^{-3} \mathrm{~V}$
The value of x is ____.