Here's a tip on how to automatically center your repeats on your shape layers:

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To create your repeat, you need a simple shape layer.
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01 - On the same layer, add a repeat effect by clicking on "add".

02 - In the offset attribute of your repeat, add the following expression: (thisProperty.propertyGroup(1).copies - 1) / -2)

03 - Duplicate the repeat effect and reverse the position from X to Y to easily create a grid.