public decimal CalculateGiftAidAmount(decimal donationAmount, EventSupplement selectedEventSupplement)
{
if (donationAmount <= 0m)
{
throw new ArgumentOutOfRangeException("donationAmount");
}
var currentTaxRate = _taxRepository.RetrieveTaxRate();
var supplementAmount = this.CalculateSupplementedAmount(donationAmount, selectedEventSupplement);
var gaRatio = currentTaxRate / (100 - currentTaxRate);
var giftAidAmount = supplementAmount * gaRatio;
return(CalculationHelper.RoundDecimal(giftAidAmount, 2));
}