I know just one rule - in the bull market you will never guess accurate top, in bear market - accurate bottom. So, it is always wisely to leave some fiat to catch next bottom.
It is true that you should have your own goal and when you achieve that price you should buy/sell and do not wait for more fall or more rise. This could actually make more losses for you as greed increases during this time and can hamper your portfolio.
it is best to buy and sell a ladder. Example: We buy Bitcoin with a correction for the price of 10,000, 9,000, 8,000, 7,000, 6,000 and then sell at a price of 20,000, 25,000, 30,000, 35,000, 40,000. I think my idea is clear