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.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.
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.
You are correct that the idea of discretionary income trumps the idea of some strict percentage to invest into bitcoin, even though both can be used as guidelines and/or even aspiration levels in regards to how much a person might want to strive to push himself to target for in his investment, but he still is going to be limited in terms of whether his target percentage is even achievable in light of his actual discretionary income.
So for example if a person knows his expenses are $1k per month, and if he is able to invest 10% of that, then that is $100 per month or $1,200 per year, or $12,000 over 10 years, so then we figure that it is going to take the guy right around 10 years to have had saved/invested 10 years worth of income into bitcoin... so that could help to target the amount and the timeline, but he still has to be able to do it.. so yeah, there cannot be contradictions, and when push comes to shove, there likely is still ONLY so much income coming in and ONLY so many expenses that can be cut.. even though people can work on attempting to being creative in terms of how they might be able to increase their discretionary income... so that it ends up being a higher percentage of their overall income/expenses... which means that they are likely to make more progress in a shorter period of time, as long as they do not end up overdoing it.
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Exactly... its pointless waiting to observe market for what?? Better chance they say which might even be a forever loop 
Bitcoin waits for nobody... observing can also be used but that should be for those already in the game and nor even new to it , it could be taking as a factor to buying more Dips which is not bad but on the other hand, I don't know what a beginner who haven't invested a dime observe
DCA and DCA and more DCA should be the main focus of a beginner .
Although, we can say a beginner wants to learn a proper way to DCA (more appropriate)
There is nothing wrong with engaging in techniques to improve DCA, which truly if someone is new to bitcoin, they might start with a certain weekly amount for their DCA, yet in the meantime, after several weeks, they could be figuring out various aspects of their budget and maybe how to employ their DCA better. On a personal level, for the first year that I was in bitcoin, I tended to give myself a weekly DCA budget (or allowance), so during the week, I would try to maximize the buying on dips within the week, however, if I had not used all of the weekly allowance by a certain time of the week, I would just buy with the rest of it at that deadline time in each week, since my new weekly allowance would become available at the beginning of the next week... but yeah sometimes going through the process, with the passage of time, I was able to figure out some ways of employing my DCA that worked better for my own circumstances... but also recalculating my situation, that surely went even beyond the first year, since the strategies became different with the passage of time (as you suggested might become the case for anyone going through their BTC accumulation journey) and the fact that I had been accumulating BTC through the previous time and the details of that prior stacking in regards to how much, what was the average costs and considering whether my stacking goals were changing and/or close to being met, helped to inform any changes in my practices.