The Iteration that is used needs an initial value which is used as a first guess for the correct implied volatility. In doubt you should enter the at-the-money-volatility. Note that the implied_vol - routine calculates the volatility for a plain vanilla Call or Put with notional 1 in foreign currency. If the implied volatility is below 0.1 % or bigger than 200%, the function returns the error code -992, if the input data are not consistent, e.g. StartVol > 100%, the function returns the error code -999.