This raises a big issue that every trader has to deal with, even after many years of trading. What you are referring to is looking at the wrong end of the straw…
It is a sad fact that no matter how good your entry criteria is, or how disciplined you are in following your rules, or how many different types of indicators or criteria you apply, you can never know how far a particular move is going to go once it has started. Every move is different and its future duration is an unknown. The only information we have is the current price and the historical prices. We do not know the future prices or the future factors that will drive it further and in which direction.
So what can we do about it?
What we shouldn’t do, when faced with a fistful of straw ends and asked to pick one, is grab the whole bunch in the hope of picking the longest straw. We will surely also pick the shortest straws as well. This is like a trend trader acting on every entry signal in order to catch the big trends but getting whipsawed over and over again while waiting for it (big trends are not that common).
It is relatively easy to design trade entry signals and the world is full of strategies how to do so. But what is not so common, and a million times harder to define, is an optimal exit strategy. There are many techniques for exits including mathematical formulas like Fibs and reversal patterns like trailing stops and MA crossovers, etc. But the facts are that it is impossible to predict future distance, and reversals tend to be faster and more furious than the build up.
So, once again, what can we do about it?
Here, I think, is one of your brain gems. In my opinion, this is not only precisely correct, it is also the only way to optimise gains and minimise losses.
Bearing in mind that we only have historical price data to work with, then we need to make two assumption:
a) price movement is not entirely random, b) a new move will usually see some degree of follow-through as other traders start to recognise the move and enter or build positions.
Based on those two assumptions, we can look at recent price movements to form a judgement of the likelihood (probability) of the possible strength of the follow-through.
@darthdimsky points to one technique which is the ATR, other will use a momentum indicator to add confluence such as RSI. Others (myself included) will use multiple timeframes to form a judgement of whether we are living through a pullback in a main trend or a reversal into a new trend - or just ranging, going nowhere!
I won’t go into details about my own approach but basically I look at the same things on a daily, 4-hour and 1-hour charts. When the signals line up on all three then I anticipate a stronger/longer move. When there is a discrepancy then, depending on my rules, I will trade a fixed pip distance target which is based on the historically typical short term moves that the instrument makes.
As usual, I am writing a “book” about something that could be said in a few lines. But basically, entry criteria alone is not enough to decide trade potential. Afterall, if the price moves even 4 pips in the “right” direction, it is a “correct” signal - but was it worth it? So you need something else to determine the likelihood of a sufficient follow-through to meet your target expectations - as well as setting your target expectations according to the likely follow-through.
As @Mondeoman rightly says, we have covered an enormous amount of material here recently and I am going to step out for a while and give you some peace to turn your thoughts into actions and to see how your trading is improving. Looking forward to seeing some positive posts in October! 
