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.
Under this circumstances, the best an individual can do is inject max 10% of their income to Bitcoin while setting up emergency fund with the remaining 10% since 80% is already expected to be used for basic needs. These are just my estimations, there could be variations.
OK, then in that situation, I truly hope that they could save enough to buy and HODL enough to make life-changing ROI, OR they learn enough about Bitcoin, the network, and its ethos, then simply hold/use it for what it is.
But practically speaking, it's very hard to have conviction to HODL, if the real world requires something, including the value saved in Bitcoin. It'd also probably why the younger generation could have the luxury of having more conviction. Because they have less responsibiities, it would be easier for them to keep large portions of their salaries without worrying for their needs in the real world.
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.
Under-employed can still do $10 per week and still be fine with his needs. That one is under-employed does not mean he has no way of saving a little, just that such a person cannot do certain things that money can do because of financial limitations but nothing beats humble beginning. As a matter of fact, they even need the investment more if they have to remedy their situation.
Good luck to you, ser and I truly hope you make it.