Perhaps you're probably correct, about your assumption that how much of your discretionary or disposable income is what determines how much you should invest rather than how much you earn, but I'd like to point out to you @Tmoonz that you're wrong from a different view point; bitcoin investment is not a default option in which you must invest what you have at your discretion; it's an important choice that should be taken seriously.
Perhaps when one talks about investing in bitcoin, it is supposed to be a very important part of one's life that is worth cutting other expenses to increase the rate and/or level of your investment; your lifestyle can wait for a moment, but bitcoin cannot, so you must sometimes not make yourself too comfortable by investing a small portion of your income in bitcoin, instead take it upon yourself to make a better portion of your income count by investing it in bitcoin. For instance, if you earn $200 per week and your weekly spending for the so-called lifestyle is $190, and you have roughly $10 in discretionary or disposable income, is that a good investment? Absolutely not! So it's basically how much you earn and a percentage you're willing to give for the future, rather than what's in your disposable income, because bitcoin isn't like shitcoin, where you can put whatever you want, but you decide how much of your income should be invested in your long-term investment scheme.
I think it's not a problem, okay maybe it would be great if we could minimize the expenses that we will do because after all it becomes a plus because with that we can be more free to be in bitcoin with a little bigger but in the end we also have to realize that we don't need to force it directly because investing in bitcoin is not a compulsion and the initial benchmark is still how ready you are to lose and we also have to see how much budget we really have to fulfill consistently so that what we do in investing in bitcoin does not eat up the budget of the others.
With this in mind if indeed we have around $200 of income and we have to spend $190 for our needs then there is no problem if indeed it is only $10 because this is our ability. although indeed in terms of profit also in the end it will not be much different but if indeed we can afford it only that much then still do it because forcing it to be more can ultimately burden yourself in the future.
See this matter of investing in bitcoin is also something we should take seriously as much as we need to plan for our daily needs, we also have to understand that needs don't finish and they keeps coming up on daily basis, but as much as we have to take care of other things we have to understand that we have to take our investment in bitcoin very serious too. If someone is making a monthly income of $200 and he is investing only $10 I think it's very poor. If you are making monthly income of $200 you should at least invest 10% of the money which is $20 and I don't think it's too much. If $190 can carry you for a month, I think $180 will equally be enough to carry you for the moment. In as much as we are investing, we should also target to have a reasonable amount of fraction of bitcoin and to accumulate it, we have to make some certain level of sacrifice.