Great idea. Sounds fun.

Note that the new map wizard isn't designed to handle fractional map units (even though the program itself does), so trying to input those values would result in a map 3 by 2 units. You'll need to create it as 4 by 3 units and manually resize if you need it that exact or just accept the little bit of extra.

At that scale, you may also consider mapping in feet and not miles, but that does include a bit more of setup when starting with an overland template, so I won't recommend doing that until you get a bit more familiar

Now in the atlas. Thanks Quenten.

These maps are now in the atlas. Thanks for the contribution.

The easiest way to accomplish this is to first draw the hex grid, then move the origin to the center of one of your hexes after drawing the grid. The snap grid from the hexes will allow you to place the origin perfectly in the center of a hex.

After doing this, the snap grid will be off, but just right click the grid button, and edit the "User Hexgrid" and set the grid offset to 0,0.

Note that when doing this, the origin will only be in the absolute center of the map if your map size was set up to be a proper even multiple of hexes height and odd multiple of hexes wide, otherwise, it will be pretty close to the center, but in any case, it will be in the correct center of a hex.