I assume you mean a perpendicular distance. Lets say the disk lies in the -plane (centered at the origin) and we want the electrostatic potential along the -axis. The the distance from the disk area element to point is
The the electrostatic potential is given by
This is a straightforward integral and so the potential is given by
If you need to find the potential off the -axis then the integral is a little tricker. It is given by
where is the off-axis distance. If you need this in closed form it can be done but from the wording of the first post it sounds like you only need the on-axis solution
Hope that helps.