first of all u must have a matrix as a reference, for the number of tiles u used and the dimensions of the matrix, you can make a grid. e.g. a matrix 50*50 and 50 tiles, to position an object in the middle, the coordinated would be 25,0,25.

each tile would represent a 1 in the x axis and 1 in the z axis (in the case above). so to position an object in the middle of the first tile, the coordinates would be 0.5,0,0.5.

the way i do it is to draw the matrix + grids on a piece of paper and label the x and z axis, this will make what i'm saying easier to understand.

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