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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.
I agree with you on this, just saying we would invest an amount can put us in trouble and in most cases you could end up struggling to meet up to the amount, so it's better to make those decisions based on your discretionary income alone, prioritising bitcoin investment doesn't mean you should over do it or invest all your income into it but rather it should mean that you allocate more from your discretionary income to it or chunk down other expenses to increase your discretionary si that you can get more bitcoin and at the expense of your well-being.
Thete are also some expenses that cannot be neglected, so I'll preferably recommend expenses on enjoyments can be best reduced for bitcoin.