Can I set $Q=e$? I'm doing a Millikan's experiment question and it wants me to find the potential difference ($V_p$). So, I rearranged the equation,
$V_p={mgd\over Q}$, where $m=1.11 \times 10^{-13}$ kg, $g=9.8$ N, $d=15$ cm. 
And I just wanted to know if I can put $Q$ as $1.6 \times 10^{-19}\,\mathrm{C}$ or would I need to calculate it some other way.
 A: That depends entirely on what you are suppose to be accomplishing. 
The point of the Millikan experiment was to find $e$, so obviously he couldn't put a particular value in: if the value was know he wouldn't have to run the experiment in the first place.
On the other hand, it you were designing a piece of teaching lab kit or an experiment intended to find a improved value for the constant, you would use the known value to select the operation range of the equipment needs to have to do what is expected of it. However, you would also need to use other values as the experiment doesn't run with only one charge.
A: The Millikan's experiment involved many runs and the Q, which was a charge on oil droplets, was changing from run to run. After many runs, it was determined that the Q was always a multiple of $1.6 \times 10^{-19}\,\mathrm{C}$.
Therefore the voltage, $V_p$, required to keep the charged oil droplets suspended in the electrical field of a capacitor, would assume many different discrete values as well and those values would be consistent with the charge, i.e., $V_p=\frac {mgd} {n\times 1.6 \times 10^{-19}\,\mathrm{C}}$.
