Here's a tip on how to automatically center your repeats on your shape layers:
To create your repeat, you need a simple shape layer.
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.