duck

        float currentLength = 0;
        float midPointI = 0;
        int segmentNum = 0;
        for (segmentNum = 0; segmentNum < routeData.Count; ++segmentNum)
        {
            float previousLength = currentLength;
            Vector3 pointA = routeData[segmentNum];
            Vector3 pointB = routeData[segmentNum % routeData.Count];
            Vector3 segmentDelta = pointB - pointA;
            float addLength = segmentDelta.magnitude;
            currentLength += addLength;

            if ((currentLength/routeLength) > i)
            {
                // the i point lies between these two!
                startPoint = pointA;
                endPoint = pointB;

                midPointI = (i - (previousLength / routeLength)) / (addLength / routeLength);
                break;
            }
        }


        Vector3 routePointPosition = Vector3.Lerp(startPoint, endPoint, midPointI);