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.
How can you that to start a debate. Obviously, and logically, if the person is under-employed then his/her problem is not if/when/how to buy Bitcoin, but where and how to find his/her next job. Plus such a person would develop mental insanity with Bitcoin's volatility if all of his/her rent and shopping money is held Bitcoin. That person will be forced to sell.
By under-employment I mean people whose salary is just enough for their basic needs and little savings but not enough to afford privileges like vacation, luxuries, even buying a house as they can only rent. Such people spend as much as 80% of their income on food, rent, school fees of children and normal healthcare but not serious surgeries. People at that stage easily resort to borrowing when they have serious emergencies which do not come always though. Unfortunately, majority of workers in my country fall into this category. Minimum wage in my country is about $30 with inflation at 29%, a country of 200 million people that have over 83 million people living below $2.5 per day. So my use of that word is a reflection of the realities on ground here.
Not everyone who is accumulating bitcoin is living a comfortable life, but since they know how important it is to have bitcoin, they have adopted a strategy that will make them spend less of their money so that they can accumulate bitcoin. Anyone who is using 80% of his salary to feed will find it difficult to be consistent in accumulating bitcoin because he or she does not have enough money to take care of his financial challenges. And that person will easily sell his or her bitcoin quickly because he or she does not have enough money to keep an emergency fund that will take care of unexpected financial problems
As a general guideline, invest only funds you are willing and able to lose without experiencing significant financial hardship. This percentage will vary for each individual, as everyone's financial situation is unique. Some individuals may be comfortable allocating more funds to Bitcoin, while others may choose a smaller percentage.
By investing what you can afford to lose, you mitigate the risk of financial strain or negative consequences if the value of your Bitcoin investment declines. Prioritise essential expenses such as housing, food, utilities, healthcare, and debt repayment before considering any investments, including Bitcoin. Establishing a solid financial foundation with sufficient emergency savings and a budget that covers necessary living expenses is essential.