In actuarial notation the expression

gives the accumulated value of a yearly nominal percentage compounded daily, whether or not is positive or negative. However, in order to extract the percentage of gain or loss over an annum, the formula must change. If is positive, and given as a percentage, then your formula works. However, for negative ,

gives the annual loss as a percentage, where x is a positive percentage corresponding to -x%.

P.S. Multiplying by 100 to make "percentages" makes me cry on the inside.