Exactly.  It is good if you put some kind of a number on it, so yes, in a case like that, the hypothetical person (student) may well end up creating his DCA at $10 per week, and maybe s/he would have certain time frames in which s/he would increase the DCA for those particular weeks in which the income that ended up coming in was greater than $10.. and maybe s/he would wait for the whole month to go by in order to make sure that all of the expenses are in and that the emergency fund is sufficiently filled prior to adding the surplus income to the DCA.. and sometimes in those kinds of cases, it might be good to add the earlier months surplus to the next month, unless you end up being happy just making a lump sum investment (which would be the same as DCA, just on a monthly rather than weekly basis) with the surplus that is determined at the end of each month.

Sometimes when regular habits are made to trajectory out the expenses and the income, you will be able to determine right away if you are receiving income that would be considered surplus because if your emergency fund is already full and you have already accounted for the worse case scenario in your monthly (or even 6 month projection of income and expenses), then whenever any extra money comes in that is beyond the worse case scenario, you would be able to spend all of that extra money on bitcoin DCA or whatever else you end up choosing to use it for.. and surely in this case we are trying to focus on when you are going to know that extra income is available for bitcoin DCA and there are always going to be some other competing interests, but if you are early in your bitcoin accumulation journey then surely you may end up putting most of your extra income into your bitcoin accumulation budget and then allocating that extra income to whatever category of accumulation that you deem most fitting (lump sum, DCA and/or buying on dips).

I already told the story of my tight cashflow in 2015 and 2016 in which I had already figured out my budget with enough of an exactness that I would automatically buy BTC right away with half of any extra income that would come in during that time (still pretty early in my BTC accumulation even though I had done most of it in 2014), and the other half of that extra income would just go into my reserves.. and in order to account for whatever other expenses that I had and sometimes income coming i would have some of its own costs that might need to get plugged into my expenses.. and so for example even something as simple as selling piece of furniture or selling a car, a bicycle or a some kind of service, there might be some potential hidden cost that the half set aside would potentially cover (not an exact science, but still worked pretty well for me during that time of my bitcoin accumulation journey).