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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.
It seems to me that sometimes folks get their investment into bitcoin wrong because they might prioritize it
beyond their considerations of their disposable income.. so then they get into trouble.
Sure, there is no problem prioritizing your investment into bitcoin, but it still has to come from your disposable income rather than some abstract decision to buy $100 per week of bitcoin no matter what - and then you end up not even having enough to cover your expenses or otherwise putting yourself into a predicament that could have had easily been avoided with a tad bit more appreciation regarding your budgetary (and psychological) balancing considerations.
Perhaps the word beyond and/or over definitely will put one into some kind of trouble for doing whatsoever whether investment or any kind of thing as long as it takes the extra ordinary it could lead to some kind of predicament but in terms of investing in bitcoin, I do not agree with some folks point of investing the left over of their income into bitcoin which is in disguise put into the word disposable income whereas what they mean is absolutely an unacceptable view of investment in bitcoin to be basically from the fag end of ones daily, weekly, and/or monthly expenses which at the end might end up being some kind of garbage amount that could be less than what is necessary for an investment. But that as it is doesn't mean one should prioritize bitcoin investment to a psychological end of being inappropriate but take it as a necessity.
I really couldn't grasp the first part of your post, i understand that you are proposing that we prioritize our bitcoin investment but that in no way should be out of our disposable income, whether you like it or not our daily needs comes first before investing in Bitcoin and the best we can is to limit some of out extra expenses to get our discretionary income up and invest in bitcoin.
A disposable income doesn't or in no way is the left over of our income, its rather what is left after all our expense has been removed, and with that amount we can chose to invest it in bitcoin, you can't be insinuating that we put bitcoin investment first and then take what is left for expense, we woudl surely regret that cause we can get into unnecessary debt or emergencies due to such poor cashfkow management decision, so it's always going to be better toninvest out of our discretionary income.