Crypto currency market is also working with profit and loss like other markets. Some people are selling with a good profit, while some people are exiting the market at a loss, but I think only those who buy at the right time can make profit in the crypto currency market, because a well-timed entry can give anyone good profits. Take chances, but with proper strategy. If you are getting profit, then sell 50% of your assets.
If the price increases further, then you can sell the remaining assets with good profit. If the price falls back, you can buy back at a lower price. Just as one must find the right opportunity to buy, one must always sell at the right time to sell one's assets at a profit. No one has ever seen a perfect bottom and no one has ever seen a perfect top. So always sell your assets not just at a profit but at a reasonable profit.
Selling 50% is a no-brainer to me. You have said it yourself. No one can find the perfect bottom or the perfect top. And the thing about the perfect time to sell, can you be 100% sure that it is the perfect time? To me, the best way to buy and accumulate BTC is to DCA. (I am biased about Bitcoin, I have no faith in altcoins. Sorry in advance.) This way you can keep a balanced value of your coins. So why not do the same with selling? I am sure this will be the best strategy. At least in my opinion.
So all you need to do is do some research and analysis on the market to find the potential up point. Where it could reach in the future. Then setting a targeted sell order on that price point. Also some several others below and over that potential targeted price point. Even if your analysis doesn't meet the target, you will sell at a lower price. But you need to make sure that you are setting the price where you are making a profit. This way if the price is rejected before reaching your analyzed target, you will still be in profit. And if the price rises more compared to your analysis, you will make more profit that way. So the 50% does not work for me. I do this instead.