It depends on individuals, If someone invest 30% of his salary on bitcoin, I think no matter the emergency involved the remaining 70% will be enough for any situation that may arise
What if the person is underemployed? You started your statement well that it depends on individual and by that individual I want to believe you are referring to individual needs, his income and other factors such as his confidence in Bitcoin. We must acknowledge the peculiarity of wealth distribution, which have some people living in surplus and others barely able to feed. So their investment decision will be determined by their circumstances. The percentage should not be a factor here before some persons will draw conclusion that a particular percentage is what should be invested in Bitcoin. It depends entirely on the situation of the individual.
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Because sometimes the remaining 70% of the salary/money earned is sometimes not enough for one month's needs. Especially for an ordinary worker or employee. So if that's the case, I think a better solution is to invest Bitcoin at the end of the month. So what I mean is, for example, in month 1 you receive a salary from the company where you work. So, don't invest that money directly into Bitcoin (30%). But first use the money (100%) for your needs for a month. Then, if at the end of the month you still have money left over after that month's needs are met and the next day you will be paid again in the second month by the company where you work, then you can invest the remaining money in Bitcoin. So by using this technique I am sure it will be better and safer. Because the point is, when you invest, all your needs are met and the next day you will receive another salary. So that invested money is good cold money to invest in Bitcoin.
In the case of such person as he explained where salary is low, I don't think that even using 100% of salary to cater for the first months need would help, cause at the end of the day the next month would come with equal problems or even lesser or more still, and he would end up using the next months income to still cater for his needs, or a little would be left and he would invest that one for the next month as you said. But what about the upper month, and the next and till he continues to be in that state of low income, I think this would really affect him cause his investment would be mostly inconsistent and he might not even be able to succeed as an investor cause emergency might also occur and he would have to cater for those too, and if this inconsistency continuities it might also lead to him starting to procastinate and giving excuses which could cripple his determination and goal, what I think is best for such individual if they must invest is to remove an allocation of 10% to build up for emergency first or divide that 10% for both DCA and emergency funds, so at least they have started and they would now see the need for a salary raise to boost their investment. Cause I think so far in this thread we have come to agree that cashflow would also play an important role in one's plan to accumulate bitcoin, either a person of this level would be okay to do poorly untill he can grow his income or should not bother investing, cause he would always be at a state of emergency. While I'm not saying a person with low income cannot invest, I'm just establishing the fact that it would be too difficult for him to endure cause his progress would be slow, and his holdings would always be at risk to poor emergency funds and low cashflow, which are two major important factors we need to accumulate bitcoin successfully.