def _calculate_valuation_constant(
        lulc_cur_year, lulc_fut_year, discount_rate, rate_change,
        price_per_metric_ton_of_c):
    """Calculate a net present valuation constant to multiply carbon storage.

    Parameters:
        lulc_cur_year (int): calendar year in present
        lulc_fut_year (int): calendar year in future
        discount_rate (float): annual discount rate as a percentage
        rate_change (float): annual change in price of carbon as a percentage
        price_per_metric_ton_of_c (float): currency amount of Mg of carbon

    Returns:
        a floating point number that can be used to multiply a delta carbon
        storage value by to calculate NPV.
    """
    n_years = int(lulc_fut_year) - int(lulc_cur_year) - 1
    ratio = (
        1.0 / ((1 + float(discount_rate) / 100.0) *
               (1 + float(rate_change) / 100.0)))
    valuation_constant = (
        float(price_per_metric_ton_of_c) /
        (float(lulc_fut_year) - float(lulc_cur_year)))
    if ratio != 1.0:
        valuation_constant *= (1.0 - ratio ** (n_years + 1)) / (1.0 - ratio)
    return valuation_constant