You seem to pretty much have it on your own.  The only thing is that you're going to want to check to see if the move is possible before doing it, but your explanation otherwise seems correct.

Hello Aprone. The video you linked to was what originally inspired me to create this function in the first place. Thanks for linking to it anyways. I'm sure others can stumble across it, and find your videos to be as helpful as i do.

Good morning:

Well, you can actually calculate the center of your formation, using the fartest unit to the left, the fartest unit to the right (plus) the fartest unit above and the fartest unit below. But, if I understand, you want to mantain this formation, so, the center is not enought. Imagine your formation includes 5 or 6 units, then some of those units are not used to calculate the center and you lose their relative positions.

I suggest storing relative positions in a class or structure, "Formation", and provide it with methods to move them and transform them.
Please, be aware tat I write this code here without testing in bgt, and I haven't used bgt for a long time, so take it like a kind of roadmap.

In this example I use a vector/point class to store x and y coordinates , like:

class Vector2d
{
double X;
double Y;
}

Add some fields to your unit class:

Class Unit
{
Vector2d Position;
Vector2d FormationRelativeCoordinates;
}

Now, create a formation class.

Class Formation
{
Vector2d Center;
Member[] Members;

//Constructor
public Formation(double xCentercoordinate, double yCenterCoordinate)
{
Center.X= xCenterCoordinate;
Center.Y= yCenterCoordinate;

//Add code to initialize members array
}

}

Each time you add a unit to the formation, you should calculate its relative coordinates and store it for each unit, you can implement a method for your unit class like:

void AddToTheFormation(Formation formationParam)
{
this.FormationRelativeCoordinates.X = this.Position.X - formationParam.Center.X;
this.FormationRelativeCoordinates.Y = this.Position.Y - formationParam.Center.Y;
}

Finally, you can move your whole formation to another point implementing the corresponding method for your formation class:

void Move (Vector2d newCoordinates)
{
this.Center = newCoordinates;

//a loop foreach unit in our array, cannot remember sintax for bgt but like:
For (int counter = 0; counter <= Members.length(); counter++)
{
Members[counter].Position.X = this.Center.X + Members[counter].FormationRelativeCoordinates.X;
Members[counter].Position.Y = this.Center.Y + Members[counter].FormationRelativeCoordinates.Y;
}
}

This way you can easyly implement "zoom in" and "zoom out" for your formation, scaling like:

void Scaling (double scalingFactor)
{

//another loop foreach unit in our array, cannot remember sintax for bgt but like:
For (int counter = 0; counter <= Members.length(); counter++)
{
Members[counter].Position.X =scalingFactor * (this.Center.X + Members[counter].FormationRelativeCoordinates.X);
Members[counter].Position.Y = scalingFactor * (this.Center.Y + Members[counter].FormationRelativeCoordinates.Y);
}
}

Hope this helps.