Unity - Decrease/Increase the "Z axis" angle axis on Update()

Unity Engine
1

What is the best way to Decrease/Increase an angle without using “eulerAngles”?

I need turn my transform between 30 and 325, according to my “Y axis”.

If it is less than zero we reduce the value of “Z axis” is greater if we increase the value of “Z axis”.

This way:

I already tried:

if ( airPlanePlayerRigidbody2D.velocity.y < 0 ) {
   var rotationVector = airPlanePlayerTransform.rotation;
   rotationVector.z -= 2f;
   Vector3 vec = new Vector3(rotationVector.x,rotationVector.y,rotationVector.z);
   airPlanePlayerTransform.rotation = Quaternion.Euler(vec);
}

And this:

public void Rotate(float angle)
{
     airPlanePlayerTransform.Rotate(0, 0, angle); // this rotate forever
}
2

good day boss, im no expert actually im a noob unity programmer but i think i was able to encounter this when i was trying to follow some clony bird tutorial by quil18 below is the code he used:

float angle = Mathf.Lerp (0, -90, (-rigidbody2D.velocity.y / 3f) );
transform.rotation = Quaternion.Euler(0, 0, angle);

heres his site for the tutorial: Quill18's Unity 3d & Blender Programming Tutorials