Nether Portal Calculator

Divide the X and Z by eight and leave the Y alone. So an Overworld position of 800, 64, 400 becomes 100, 64, 50 in the Nether. Going the other way you multiply by eight instead. Only the horizontal coordinates scale, which is the part people most often get wrong.

When you walk through a portal the game looks for an existing portal near where you should come out, and only builds a new one if it cannot find anything. That search covers a wide area, so two portals that feel far apart in the Overworld can be close enough in the Nether to share a single destination. Building at the exact converted coordinates and keeping your Nether portals well separated avoids it.

Keep them more than 128 blocks apart in the Nether, which is over 1000 blocks apart in the Overworld. Any closer and there is a good chance they end up joined to the same portal.

It is not scaled, so you can use the same height, but the Nether has a much lower ceiling than the Overworld. If your Overworld portal is high up you will need to bring the height down to somewhere you can actually build in the Nether. The calculator does this for you and tells you when it has.

Because a coordinate names the block you are standing in, and the game always rounds down when working that out. It matters most on the negative side of the world, where rounding to the nearest would put your portal one block away from where it should be.