Determining the rotation of an object based on its upcoming position?

Hi,

I have an object that I would like to rotate so that it's "head" (one end of the rectangle. I want to be able to apply this to any object though) is facing the direction that it is moving in. I don't have a target that I want to look at so the object needs to be able to know it's correct rotation based on it's upcoming movements. See the following picture for an example :

My idea was to find the inverse of tan of the opposite and adjacent values ( ie x and y ) and then to convert those radians to the new rotation angle. However, this doesn't seem to give me the desired result?

Here is the code that I'm using:

         float z = Mathf.Rad2Deg * Mathf.Atan(x / y);

         GetComponent<Rigidbody>().rotation = Quaternion.Euler(0,0, z);
         

My object is moving in small increments vertically and horizontally based on the 2D plane every frame so I was thinking of doing something like so to make it seem like a more realistic rotation but it's also not giving me the result that I'm looking for. What would you recommend?

      float inc = Mathf.Rad2Deg * Mathf.Atan(x / y);

         if(inc > 0) { 
         for(int i = 0; i < inc; i++) {
             z = i;
             GetComponent<Rigidbody>().rotation = Quaternion.Euler(0,0, z);
         }
         }
         else
         {
             for (int i = 0; i > inc; i--) {
             z = i;
             GetComponent<Rigidbody>().rotation = Quaternion.Euler(0,0, z);
         }