Better to buy whenever you have extra money rather than regretting not buying earlier.
We always want to buy cheaper and sell more expensive, as OP advises. In practice, it is difficult to guess with this and it is unreasonable to expect the best price for buying and selling. As for me, this is how you should buy at the first opportunity that has fallen out and, in the same way, sell if it makes a profit. You need to proceed from your expectations, goals and strategy.
you will only rely your instincts on this matter. as no one can precisely predict the future, you will only base your decision from its history. you can also get insights on various threads of discussions but it doesn't mean, you need to follow them, but will give you enlightenment on certain matters.
also, buying cheap may be true to btc, but when it comes to other alts, you will likely be screwed as most of them will continuously to go down without the hope of reviving their market price. but when you buy btc, you should also make sure that it is not your last money as you may end up selling it below your buying price if you have urgent needs.