Nether Portal Calculator
This calculator allows you to calculate where you should place the a Nether portal to connect to an activated portal in the Overworld. Enter the XY coordinates of the origin portal in the Nether and Overworld, as well as the coordinates of the new portal in the Overworld. The resulting coordinates will be where the portal should be constructed in the Nether.
The Nether portal doesn't have to be exactly at that location, just fairly nearby.