We’re talking about percentages of a price move, from swing low to swing high, or vice versa. You can’t know the extent of the move until it’s over. Therefore, you can’t define the first 20%, the middle 60%, or the last 20% except in retrospect.
Bernard Baruch’s statement was not about the mechanics of managing a trade that he was in – it was a statement about the typical (statistical) result of his trading.
To put it a different way, you can enter an uptrend at the swing low, only if you are clairvoyant. Baruch wasn’t, I’m not, and I’ll bet you’re not, either. Therefore, the best you can do is to join a trend after it’s clear that it has begun. How much of the early trend passes you by, before you join it, will depend on many things, including your method for analyzing the market, and your degree of caution (or impulsiveness).
As I understand what Baruch was saying – overall, on average, he joined trends late (the first 20%), and he exited either (1) early, or (2) after the trend had retraced (the last 20%). So, on average, his net profit from trend trades successfully executed was about 60% of the entire move (the middle 60%).