Calculation of optimal current density

I'm trying to figure out what my optimal current density would be in respect to the limiting current density. 


How do you calculate concentration polarization and limiting current density in an electrolytic cell. There are two copper electrodes and a $2\:\mathrm{M}$ copper nitrate solution. The average current density was $0.375\:\mathrm{A\:cm^{-2}}$.


I know the Tafel expression ($\eta a=a+b \log i$) can calculate the voltage drop but can I find the limiting current density just like that in the image? 


![enter image description here](https://i.stack.imgur.com/gVNZU.jpg)


