When making models in blender for grid snapping i always use the 100x100 in odd numbers rule on the ground, but whatever height i want for the height.
When making grid snapping models you have to remember that the grid in gameguru is 100x100, so you are always working outward from a 100x100 square, from that 100x100 you work out wards in odd numbers so that the center of the model is always at the center of a 100x100 square, so you would build 100, 300, 500, 700 and so on, working like this keeps it on the 100x100 grid and allows you rotate pieces around each other and apply grid snapping.
The walls i always make a thickness of 10 for outside walls, and 5 for inside walls.
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