Mathf.InverseLerp

public static float InverseLerp (float a, float b, float value);

Description

Calculates the linear parameter t that produces the interpolant value within the range [a, b].

using UnityEngine;
using System.Collections;

public class ExampleClass : 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);
    }
}

See Also: Lerp.