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So the best time for one to have such mindset that of buying back is when they have already gotten themselves some good amount of Bitcoin stashed (having enough Bitcoin in their portfolio), like for instance a guy accumulation goal was for him to have 5 BTC , so after accumulating for some time he endup hitting that goal , so now he decide to continue with his holding though has done with the accumulating part . Then lateron he decided to start taken some profit from his holding, so while doing so he saw that his Bitcoin Stash us kinda reducing due to the withdrawing, so he may decide to buy some quantities using lump-summing or other convenience method, to purchase some quantities in order to coverup for those withdrawal he has made back then . It is not relevant that it must be same amount he withdrew, he might purchase quantities that's not up to the withdrawal or quantities that's more than that they have withdrew from Their holding .
It seems to me that in your example of having a goal of getting to 5 BTC, there might be some recognition of the ability to sell and/or to play around with the extra BTC if that same person had actually gotten to 6 BTC, so then the extra 1 BTC ends up being a overaccumulation cushion... that allows for more freedom.. so then maybe the 5 BTC becomes somewhat untouchable, yet the extra BT are able to have more flexibility in terms of whether some of them might be sold at various points.
Sure, 5 BTC might be enough too for a person who might be considering about 4 BTC is enough .. so 5 is more than enough..
And, so reassessments can be made at various points in regards to how much is enough and how to calculate how much of the stash might have more flexibility in terms of potentially setting various sell price points or even selling certain quantities on regular bases on on timeline and with formulas that you might calculate yourself. My sustainable withdrawal thread goes over timeline and also price-based sustainable withdrawal considerations.
Having more than enough Bitcoin is the dream of all investors, just like you said that an investor planned to accumulate 5btc but was able to accumulate 6btc, it means that he can do whatever he like with the extra 1btc and it will be fine if he sells in some of it whenever he wants. He can still use his 1btc to serve as his emergency funds so that he only sells when a real emergency comes, so that he will not be carried away of selling too much.