for this, i am guessing you want to use Int(E.dl).
you will use the calculator to do this.
but you will need a line to perform the integral on. before you simulate, draw a line on the region you are interested in finding the voltage difference.
i have never done this before. so i do not know if it works. this idea came up, because i had to do it at one point, but i decided not to, because i did not really need the voltage.
btw, i would not trust the voltage calculation you get.
i am interested to know if it works.
also, if you can find another relation between voltage and volume or surface, try that.