Mathf.InverseLerp  

var walkSpeed = 5.0;
var runSpeed = 10.0;
var speed = 8.0;

function Start() {
// parameter is now 3 / 5
var parameter : float = Mathf.InverseLerp(walkSpeed, runSpeed, speed);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public float walkSpeed = 5.0F;
public float runSpeed = 10.0F;
public float speed = 8.0F;
void Start() {
float parameter = Mathf.InverseLerp(walkSpeed, runSpeed, speed);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public walkSpeed as single = 5.0F

public runSpeed as single = 10.0F

public speed as single = 8.0F

def Start():
parameter as single = Mathf.InverseLerp(walkSpeed, runSpeed, speed)