"Binary" Search for a double in C#. Uses difference from needle for midpoint as opposed to halving sum

BinarySearch.cs

private static int BinarySearch(List<double> haystack, double needle, double tolerance = 0.0000001)
{
    var left = 0;
    var right = haystack.Count - 1;

    while (needle <= haystack[right] && needle > haystack[left])
    {
        var leftDiff = needle - haystack[left];
        var rangeDiff = haystack[right] - haystack[left];
        var count = right - left;
        var range = (int)(leftDiff/rangeDiff*count + left);
        if (needle > haystack[range])
        {
            left = range + 1;
        }
        else if (needle < haystack[range])
        {
            right = range - 1;
        }
        else
        {
            left = range;
        }
    }
    if (Math.Abs(needle - haystack[left]) < tolerance)
    {
        return left;
    }

    return -1;
}

[TestMethod]
public void BinarySearchTest()
{
    var test = new List<double>
    {
        0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19
    };

    for(var i = 0; i < test.Count; i++)
    {
        Assert.AreEqual(i, BinarySearch(test, i));
    }

    test = new List<double>
    {
        0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20
    };

    for (var i = 0; i < test.Count; i++)
    {
        Assert.AreEqual(i, BinarySearch(test, i));
    }
}