Minecraft Map Tool FAQ: converting Overworld and Nether coordinates (divide or multiply X and Z by 8), linking Nether gates, managing markers, and backups.
Use the Overworld ⇔ Nether coordinate converter on the map page. Nether X and Z are the Overworld X and Z divided by 8 (rounded down, as in the game); Overworld X and Z are the Nether X and Z multiplied by 8. Y stays the same. Type on either side and the other side updates. On the map, the panel also shows the equivalent X and Z as you move the cursor.
Create one marker in Overworld and one in Nether, set both to Category: Portal and Type: Nether Gate, then edit either marker and select the counterpart in Linked Nether gate.
After saving, use Jump linked gate from the marker list to move to the linked marker in the other dimension.
Candidates are shown only when Type is Nether Gate and only for gates in the opposite dimension. End markers are not eligible.
Use Export JSON to download your local data, then open the tool in another browser and use Import JSON to restore it.