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.
It is in the planning that the amount to be invested into be Bitcoin will be calculated. The planning is a critical stage because it covers the most important aspect of the investment process, enabling the investor to follow the best course of action.
I believe another critical stage is buying the DIPs of the bear market. It was there during 2019 and again during 2022. Currently if you are under-employed, it's probably better for you to save what you can and wait for a other cycle. OR you could get lucky and a Black Swan happens.