Get the gameobject to the left/right of current target, in field of vision?

I'm trying to write the math to cycle between lockon targets. Basically the player is currently locked onto one target (looking at them, the target has an eye over its head etc etc etc). If the player hits left on rightHorizontalStick, it should switch to the next viable target to the left of the current one, and same for if they hit right. The problem is that rather than just being left/right on one axis, it's relative to the curve that is made up by the camera's vision. Depth does not matter at all, only left/right position along this curve

Here are two picture to better communicate what I mean (the black dots are other enemies):

http://i.imgur.com/vqA3a6S.png

http://i.imgur.com/7f0keyD.png

How could I get the next dot to the left/right of the blue dot?

Here is the code from my attempt. Idk if I just need to tweak the math or if there is an entirely different, much better approach, but just in case, I figured I should post what I did myself

 private void changeLockOnTarget(direction d)
 {
     float yRot = player.transform.eulerAngles.y;
     float distanceFromCurrLockOn = Mathf.Infinity;
     GameObject newTarget = null;
     GameObject[] allEnemies = GameObject.FindGameObjectsWithTag("Enemy");
     
     foreach(GameObject enemy in allEnemies)
     {
         //if not current target && enemy is in camera view
         if(enemy != lockOnTarget && GeometryUtility.TestPlanesAABB(GeometryUtility.CalculateFrustumPlanes(mainCam), enemy.collider.bounds))
         {
             float distance;
             if(d == direction.left)
             {
                 float xAmount = (lockOnTarget.transform.position.x - enemy.transform.position.x) * Mathf.Cos(yRot * Mathf.Deg2Rad);
                 float zAmount = Mathf.Abs(lockOnTarget.transform.position.z - enemy.transform.position.z) * Mathf.Sin(yRot * Mathf.Deg2Rad);
                 
                 distance = xAmount + zAmount;
             }
             else if(d == direction.right)
             {
                 float xAmount = (enemy.transform.position.x - lockOnTarget.transform.position.x) * Mathf.Cos(yRot * Mathf.Deg2Rad);
                 float zAmount = Mathf.Abs(enemy.transform.position.z - lockOnTarget.transform.position.z) * Mathf.Sin(yRot * Mathf.Deg2Rad);
                 
                 distance = xAmount + zAmount;
             }
             else distance = 0; //this will never happen
             
             if(distance < distanceFromCurrLockOn && distance > 0)
             {
                 Debug.Log("newTarget is now " + enemy.name);
                 distanceFromCurrLockOn = distance;
                 newTarget = enemy;
             }
         }
     }