Formula for scaling an object according to how far it moves from an original point and viewpoint, in order to maintain its perceived size. In the form of a Unity MonoBehaviour. This is to achieve VR-friendly parallax for 2D games.

DistanceApplier.cs

using UnityEngine;

/// <summary>
/// This script will push an object back to a target Z position and maintain its size to a given editor camera distance. This is to achieve parallax that is compatible with VR.</summary>
public class DistanceApplier : MonoBehaviour {

	public float targetZ = 20;

	// The applied scale will try to maintain the scale of the object assuming the original scale was defined when viewed with the camera at this z position.
	const float BaseCameraZ = -10;

	void Awake () {
		Vector3 originalPosition = transform.localPosition;
		float originalDistance = BaseCameraZ - originalPosition.z;
		float newDistance = BaseCameraZ - targetZ;
		float uniformScale = originalDistance == 0 ? 0 : newDistance / originalDistance;

		transform.localPosition = new Vector3(
			originalPosition.x,
			originalPosition.y,
			targetZ
		);
		transform.localScale *= uniformScale;
	}
}