        if (_params.surplusTransferPercentage < ONE_HUNDRED_WAD) {
    _amountToSell: _params.surplusAmount.wmul(ONE_HUNDRED_WAD - _params.surplusTransferPercentage)
WAD = 1e18
surplusTransferPercentage = 1e18 (representative of 100%)
surplusAmount = 3e18
ONE_HUNDRED_WAD = 100e18
if (_params.surplusTransferPercentage < ONE_HUNDRED_WAD)
    _amountToSell: _params.surplusAmount.wmul(ONE_HUNDRED_WAD - _params.surplusTransferPercentage),

Returns 297e18; where coin surplusAmount is only 3e18:
_amountToSell = 3e18 * (100e18 - 1e18) / 1e18
              = 3e18 * 99e18 / 1e18
              = 297e18
              = 297 WAD
