Formation of Units doesn't account for rotation

Hello,

I am having issues with a formation script I have been mucking around with. Essentially, I am looking to make a basic formation system similar to the Mount & Blade games, where the units move into position in a grid behind the player or at a specific point and this grid can be dynamically adjusted (number of rows, number of units in the formation, etc.)

Most all of the script is working as I intend but when the primary point rotates, the units don't rotate around the back of the object.
I have attached a picture to better explain

The following code is in Update and builds the grip for the units to move to. I believe the issue is with the X and Z positions. I'd need to have them adjust to reflect the rotation of the RallyPoint but I cannot for the life of me think of how I'd do that. Sorry if the answer is obvious

 Vector3 formationStartPosition = rallyPoint.transform.localPosition - rallyPoint.transform.forward;
 
         for (int i = 0; i < units.Length; i++)
         {
             for (int y = 0; y < rows; y++)
             {
                 if (i % 2 == 0)
                 {
                     positions[i] = new Vector3(formationStartPosition.x + (seperation * i), formationStartPosition.y, formationStartPosition.z);
                     
                 }
                 else
                 {
                     positions[i] = new Vector3(formationStartPosition.x + (seperation * i), formationStartPosition.y, formationStartPosition.z - seperation);
                 }
             }
         }

Second Image of the script running currently. Capsule is the RallyPoint and Squares are the Units. Can someone offer any advice on what I could do? Thanks!