Always make sure the GrooveCenter is properly set up and zeroed. Look here for instructions

How to widen a centered slot We need to make a centered slot in this board. We think it’s going to be a ¼” wide slot, so we set up the router fence for that.
How to widen a centered slot As always, drop the GrooveCenter in the router Arbor and tighten the Carriage to enclose the board. The Fence Pin indicates where the fence goes.
How to widen a centered slot Once the fence is securely positioned, we chuck in the ¼” router bit and start the operation.
How to widen a centered slot Here is the result: a nicely centered slot.
How to widen a centered slot It turns out that ¼” wasn’t enough: the slot needs to be widened to 3/8”. Now it’s even more important that the operation stays exactly centered. But the fence has been moved in the meantime and we have to find its exact same setting! We certainly don’t want to do two passes: one of them would be a dangerous cut with the work piece between the router bit and the fence, or otherwise a least a climb cut if we come in from left to right. But the GrooveCenter sets up for a perfectly centered cut every time, whatever the router diameter or the board’s width!