Start from the existing portal
Write down the X and Z of the portal you intend to keep. Divide those values by eight when going to the Nether and multiply them when going toward the Overworld.
Pick a buildable location
Treat the converted X and Z as the target. Choose a safe Y level near that point with space for the frame and safe access on either side.
Make a round trip
Enter the new portal and return through the one you land in. If either direction connects to another portal, record all nearby portal positions and check which is closest to your intended destination.