In the case of such person as he explained where salary is low, I don't think that even using 100% of salary to cater for the first months need would help, cause at the end of the day the next month would come with equal problems or even lesser or more still, and he would end up using the next months income to still cater for his needs, or a little would be left and he would invest that one for the next month as you said.

But what about the upper month, and the next and till he continues to be in that state of low income, I think this would really affect him cause his investment would be mostly inconsistent and he might not even be able to succeed as an investor cause emergency might also occur and he would have to cater for those too, and if this inconsistency continuities it might also lead to him starting to procastinate and giving excuses which could cripple his determination and goal, what I think is best for such individual if they must invest is to remove an allocation of 10% to build up for emergency first or divide that 10% for both DCA and emergency funds, so at least they have started and they would now see the need for a salary raise to boost their investment and manage the rest 90% for needs and expenses,  such person must also have a high discipline more than another investor starting with higher capital and comfort  to manage his cashflow and chunk some unnecessary expense out to help himself.

Cause I think so far in this thread we have come to agree that cashflow would also play an important role in one's plan to accumulate bitcoin, either a person of this level would be okay to do poorly untill he can grow his income or should not bother investing, cause he would always be at a state of emergency.

 While I'm not saying a person with low income cannot invest, I'm just establishing the fact that it would be too difficult for him to endure cause his progress would be slow, and his holdings would always be at risk to poor emergency funds and low cashflow, which are two major important factors we need to accumulate bitcoin successfully.