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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 $100 per week of bitcoin no matter what - and then you end up not even having enough to cover your expenses or otherwise putting yourself into a predicament that could have had easily been avoided with a tad bit more appreciation regarding your budgetary (and psychological) balancing considerations.
Perhaps the word beyond and/or over definitely will put one into some kind of trouble for doing whatsoever whether investment or any kind of thing as long as it takes the extra ordinary it could lead to some kind of predicament but in terms of investing in bitcoin, I do not agree with some folks point of investing the left over of their income into bitcoin which is in disguise put into the word disposable income whereas what they mean is absolutely an unacceptable view of investment in bitcoin to be basically from the fag end of ones daily, weekly, and/or monthly expenses which at the end might end up being some kind of garbage amount that could be less than what is necessary for an investment. But that as it is doesn't mean one should prioritize bitcoin investment to a psychological end of being inappropriate but take it as a necessity.
You are still struggling a little with what a discretional income is, investing from our discretional or disposable income doesn't mean in any way that we are neglecting our investment in bitcoin and doesn't mean that its some kind of left over or little amount.
A discretional income is what is left after we have taken out the money for our necessary expenses, I think i already gave you an illustrative the last time but I think I'll have to start all over,
Mr A is earning a 1000$ and lives in a small house with little bills to pay, so after he has taken out his necessary bills and expenses he ends up with 500$ in his discretionary income and can decide to keep an extra 100$ for floats and invest the rest into bitcoin on a weekly basis and this is achieved because he has little expenses to make and hence his discretionary income is bigger after he has removed the necessary expenses.
Mr B can also be earning same amount and has a higher expenses based on the kind of house he chose to live and lifestyle he has so he winds up with only 300$ as a discretional income after after his expense and decides to keep an extra 150 floats for enjoying himself and invest 150$ into bitcoin.
So in essence the discretional income is determined by how much of an expense the person has and how much he can avoid out of his income, and at same time it's also possible that the person can chose to favour other things than bitcoin investment with his discretionary income.