You are right mate, you should have a fixed and pre-planned target % before you even make a trade - this is ideal because it removes the emotional aspects in trading most especially if the price goes down or goes up. You just got to set a certain target gain, open a sell position of the said gain and logout and only check it every couple of hours so you won't get affected emotionally of the price volatility.
Well.its not gonna hurt all the time if you become a little flexible with your target. It's good that you have a fixed target amount for selling but sometimes, if you're not the type of person who easily panics, its good if you can wait a little more and just use that target amount as basis for your selling rate