function calcBonus(uint256 tokenAmount) internal view returns (uint256) {
        uint256 bonus = 0;
        if (_kickPurchased.add(tokenAmount) <= _bonus20capBoundary) {
            bonus = tokenAmount.mul(20).div(100);
        } else if (_kickPurchased.add(tokenAmount) <= _bonus10capBoundary) {
            bonus = tokenAmount.mul(10).div(100);
        }
        return bonus;
    }