function _withdrawTo(uint _deposit, uint _withdrawn, uint _blockTimestamp, uint _total) constant returns (uint) {
        uint256 fraction = availableForWithdrawalAt(_blockTimestamp);

        
        uint256 withdrawable = ((_deposit * fraction * _total) / totalfv) / precision;

        
        if (withdrawable > _withdrawn) {
            return withdrawable - _withdrawn;
        }
        return 0;
    }