Is there a way to slice a map PERFECTLY through the center, or do you have to guess where the dead center is?

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Map coordinates are shown on the bottom status bar of WorldBuilder, they show the coordinates of where your cursor is pointing at. If you know your map size, you can find the center by finding the correct coordinates.

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or just put a worldbuilder into the upper right corner (inside boundary), then click on it and half the x and y coordinates, then you get a worldbuilder in the middle.