Well it depends....(doesn't everything?)......just closing out half the position at half the target, or whatever, is a bit crude and rigid.
Personally I like to keep a flexible approach so depending on the overall position size and how you see the market as it approaches the target price you may decide to cut some/all of the position there, or move up/down your target, or take a bit of a flier with the last 10% of the position, etc.
My general philosophy is that there is little point in having ONE single unique price that you decide to buy/sell at. If you do, are you not in effect claiming that that price is the highest/lowest that will be achieved in that move? e.g. The market is at 50 and you decide you want to sell if it hits 60 so you put in an order to sell your whole position at 60. But isn't there are very good chance it could go to 61, 62,...65 before it plummets to 10 as you predict? Even worse it just touches 59 and you don't get filled before it plummets to 10. (and of course it works exactly the same when exiting a position) In this example I would perhaps sell some at 55, some at 58, 60, 62 and 65. Obviously if you are trading one lot that's pretty impossible.
FTSE B
I can't quote or even point you towards the mathematical proof but intuitively it does make a lot of sense, doesn't it? How many times has the market missed your target by just a tick or two, then gone on to move in the way you predicted without you 'on board'.? Or it just misses your profit target by a tick and instead of a healthy profit you are out for flat (or even a small loss).
So it is psychologically enhancing as well as profit maximising.