Since there didn't seem to be any readily available, I decided to put these together myself based on my own experiments and some help from agentorangeguy. If there is something that seems to be wrong, do not hesitate to point it out.
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Ultima 6 Mod for Exult site: http://www.ultima6.realmofultima.com/
I must say that using this chart is way better than converting from hex to base 12 then using the base-12 digits to locate the ireg file on the map.
What, that is how I did it before...
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Marzo Sette Torres Junior
aka Geometrodynamic Dragon
[url=http://www.catb.org/~esr/faqs/smart-questions.html]How To Ask Questions The Smart Way[/url]
Doesn't look like these have been added to the Studio documentation so far. Just a nudge.
Also, this grid should be applicable to any custom Exult map. I scaled these from 200x200 pixels to 600x600 so they can be neatly divided by twelve. If anyone wants, I can easily make an image with just the grid to help overlaying, if anyone wants that.