How to spawn objects on a restricted sphere surface relative to camera?

Thanks elenzil, that helped me arrive at working solution.

This is what I came up with:

     private Vector3 generateTargetPosition() {
         float currentPhi = 180 - (centerEulerRotationRange(Camera.main.transform.localRotation.eulerAngles.x) + 90);
 
         //radial distance r, polar angle θ (theta), and azimuthal angle φ (phi)
         float r = targetDistance;
         float theta = Random.Range(-targetRelativeHorizontalFOV/2, targetRelativeHorizontalFOV/2); // Random horizontal angle change relative to facing
         float phi = 90 + Random.Range(-targetAbsoluteVerticalFOV/2, targetAbsoluteVerticalFOV/2) + (90 - currentPhi); // Random vertical angle change relative to horizon angle
 
         theta *= $$anonymous$$athf.Deg2Rad;
         phi *= $$anonymous$$athf.Deg2Rad;
 
         float x = r * $$anonymous$$athf.Sin (phi) * $$anonymous$$athf.Cos (theta);
         float y = r * $$anonymous$$athf.Sin (phi) * $$anonymous$$athf.Sin (theta);
         float z = r * $$anonymous$$athf.Cos (phi);
 
         Vector3 targetOffset = new Vector3(y, z, x); // Swap coordinates around to match unity coordinate system
 
         Vector3 targetPosition = Camera.main.transform.TransformPoint(targetOffset);
 
         return targetPosition;
     }
 
     // eulerAngles.x ranges from 90 (down) to 0 (forward), 360 (forward) to 270 (up)
     // This function transforms the range to -90 to 90
     private float centerEulerRotationRange(float ang) {
         if (ang >= 0 && ang <= 90) {
             return -ang;
         } else if (ang >= 270 && ang <= 360) {
             return 90 - (ang - 270);
         } else {
             throw new UnityException("I don't know how eulerAngles work");
         }
     }