You have to attempt to use your best judgment. If you have absolutely no savings, then surely you are starting from zero, yet we know that to invest into bitcoin at all you have to have some reasonable ability to assess and conclude that you have discretionary income, and by definition, discretionary income is money that is left over after you have accounted for all of your expenses. If you are trying to invest into bitcoin from money that you need for expenses, then you do not have discretionary income and so you should not be investing into bitcoin.. because you would merely be taking chances with money that you actually need to cover expenses.
For practical purposes, I frequently like to presume that even the poorest of people will tend to try to have some kind of a cash cushion to try to cover 2-6 weeks of their living expenses in case their income really were to dry up.. so in some sense, if a guy does not even have 2 weeks of living expenses that are saved up, then he may well not even have enough discretionary income to be able to justify investing into bitcoin.
So, there is a need to establish that you have discretionary income to even justify investing into bitcoin at all, and so even though people can do whatever they like, I don't recommend investing into bitcoin if you do not have discretionary income or if you are not sure... If you are not sure, then it is better not not invest into bitcoin with money that you may well need for expenses.
Once you figure out that you have discretionary income and you decide that you want to invest into bitcoin, then frequently I suffest that at minimum, you should be striving to protect your bitcoin investment by simultaneously building your emergency funds and your investment into bitcoin at the same rate until you get up to 3 months of expenses and the equivalent of three months of your expenses invested into bitcoin.. and sure the bitcoin side of the matter may well go up and down in value more than the part that you are keeping in fiat... so if you ONLY have $10 per week of discretionary income that you are able to invest into bitcoin, you may well have to invest half of that $5 into your emergency fund and $5 into bitcoin.
Definitely true , without discretionary income There's no need to invest in Bitcoin . Because if one start without any discretionary income he or she is just trading or rather gambling , because using money that's meant for taken care of expenses will only lead to one running back to his Bitcoin inorder to handle those expenses. And the funny thing he might even sell in loss due to price fluctuations (price wait for no one ) , so he might be unfortunate to sell when price drop .
Better to start small if your discretionary income is $15 in two week, you can choose to set the two week as your dca fix time of purchasing because it can be either weekly or monthly. So you can focus $10 as your accumulation funds while the remaining $5 as your emergency funds so that you can handle your expense without running to Bitcoin.