100Destination X
64Destination Y
-25Destination Z
×8 / ÷8Horizontal only
Why two portals still dump you in the wrong base
The game first converts X and Z by 8, then looks for an existing portal near that ideal point. If another portal is already inside the search window, you get that one instead of a new link. Build the destination portal at the converted coordinates before lighting the source, or break the extra portal that is stealing the link.
Which rounding does this use?
Java floor: 29.9 becomes 29, and −29.9 becomes −30. That matches vanilla Overworld → Nether conversion. Y is copied unchanged.
Java vs Bedrock search radius
Going to the Nether in Java, the search radius is 16 blocks. Going to the Overworld it is 128. Bedrock uses 128 in both directions. Always light both sides if the bases must stay 1:1.