There are a few factors that can effect the Flex temperature. If the calculated Flex is 67 degrees from performance chart using your TOW, we take in factors if the runway conditions are wet (usually reduces the flex temperature), also the QNH plays a part if QNH is anything but 1013 it changes the flex temprature as well. So the end result may not be 67 but it could be lower (if wet and low QNH).
Now there are certain limitations when you can or cannot flex. I'll be more than happy to type it out for you if you need them.