Divide Rectangle With Given Values?

//So start with having a list of rectangles
//this is to keep track of all the rectangles you're creating within your method)
List<Rectangle> RectList = new List<Rectangle>();
public void CreateRectangles(int numberofrects)
{
    // when calling this method, you put in how many rectangles you want (INCLUDING) the first one of 600, 600
    Rectangle OldRectangle = new Rectangle(600, 600, 100, 100);
    for (int i = 0; i < numberofrects; i++)
    {
        Rectangle NewRectangle = new Rectangle(OldRectangle.X + (100), OldRectangle.Y + (100), OldRectangle.Width -= (100), OldRectangle.Height -= (100));
        RectList.Add(NewRectangle);
        OldRectangle = NewRectangle;
    }
}