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.
Getting some sort of prediction for the future is impossible if you are looking for something that is exact in the end. I believe that we are going to end up with a good return if we end up selling our capital back and then just deal with the profits.
Think about it this way, you could just sell the capital out, and the rest would be amount you risk as much as you want, if you want do the most outrageous risky thing you can think of, and then you could do whatever you want to do with it. That is the most important part, always trying to do something that you could limit your risk that way and no matter what you would do be able to profit without thinking about what could go wrong. This is a method that helps people for the long term and could make you go towards the right direction.