How do I link a Nether portal to the right place?
Stand where you want the Overworld portal, press F3 (Java) or turn on "Show coordinates" (Bedrock), and type the X, Y and Z here. The calculator divides X and Z by 8 and rounds down; Y stays the same. Build your Nether portal on that block and the two portals pair up instead of the game creating a new one somewhere you did not choose.
Why divide by 8?
One block in the Nether stands for eight blocks in the Overworld, horizontally. Walking 100 blocks in the Nether moves you 800 blocks in the Overworld, which is why players use Nether tunnels as fast travel. Height is not scaled, so the Y value is copied over, but keep the Nether's own limits in mind: the bedrock ceiling and the lava sea decide where you can actually build.
Why does a negative coordinate round "the wrong way"?
Coordinates name the block whose corner is at that number, so −1 ÷ 8 lands in block −1, not 0. The calculator rounds down like the game does, which is why −357 becomes −45 and not −44. The exact decimal result is shown under the block so you can check a position you read off the F3 screen.
How far does the game search for an existing portal?
When you step through, the game looks for a portal near the converted point before it builds a new one. Java searches 128 blocks around the point when you arrive in the Overworld and 16 blocks when you arrive in the Nether; Bedrock searches 128 blocks in both. If two portals sit inside that range, the closest one wins, so keep separate portals far enough apart in the Nether, or they will all lead to the same place.
Does this work in multiplayer and on servers?
Yes. The rule is part of the game, not a client setting, so it applies on Realms and on any server that has not changed the Nether's scale with a plugin or data pack. If your server's portals land somewhere else, ask its owner whether a portal plugin is installed.