How can I offset blocks in a tower to build a Jenga-like structure?

I'm to the halfway point getting this worked out. Independently, if I build rows that face a singular direction the tower is generated fine. However, as soon as I rotate them - in this case via odd numbers, the tower falls apart... or just explodes. I know I need to place an offset on the x position, but I'm not sure what the best calculation for that would be. The tower structure is built from Vector3.zero in the world.

     void buildTower() {
 
         int counter = 0;
 
         for (int y = 0; y < numY; y++) {
             for (int x = 0; x < numX; x++) {
                 Vector3 pos;
                 Quaternion rot = Quaternion.identity;
 
                 Vector3 rotEuler;
 
                 //rotEuler = new Vector3(270, 0, 0);
                 //pos = new Vector3(x, y, 0) * spacing;
 
                 if (y % 2 != 0) {
                     rotEuler = new Vector3(270, 0, 0);
                     pos = new Vector3(x, y, 0) * spacing;
                 } else {
                     rotEuler = new Vector3(270, 270, 0);
                     pos = new Vector3(x, y, 0) * spacing;
                 }
 
                 rot.eulerAngles = rotEuler;
 
                 GameObject plank = Instantiate(plankObject) as GameObject;
 
                 plank.transform.parent = Tower.transform;
 
                 plankObject.transform.position = pos;
                 plankObject.transform.rotation = rot;
             }
         }
     }