Engaging in bitcoin investment first starts from the mind. A person who understands how important it is to own a bitcoin will own a bitcoin as long as he is willing to Spare little for bitcoin. Discretionary income can come at anytime through any possible means through which the individual has been sustaining himself even though the source is not stable. He would understand that no matter how he tries not to invest any part of that money in bitcoin because he feels it's not enough, the money would definitely finish. So any moment there's an available discretionary income he can invest a part in bitcoin and condition his mind as though he had spent the money. This would help him to forgo the bitcoin and instead of stalking his bitcoin wallet, he would channel more energy into getting a stable source of income in order to boost his bitcoin investment.

People who live with a scarcity mentality would never invest in bitcoin at anytime because they would always believe that what they have is not enough to sustain them not to talk of investing in bitcoin. This has kept many people away from owning any reasonable investment even when they finally get a stable source of income. Man's want increases with increased income and until you separate your Needs from your wants, you would never make any progress.  Sparing $20 From your discretionary income would already make you a bitcoin owner. So why wait further? Bitcoin is an asset that is worth investing in so everyone who can sustain himself has no reason not to own a bitcoin.

Yeah sure they are two different things but I am pretty sure that everyone knows when he has a particular amount that he doesn't necessarily need at the moment. Aside from allowing too much encroachment from human wants we will discover that we have more discretionary income than we admit having. So it is very important to be intentional about owning bitcoin because no matter how much comes into your account today, you will always see reasons to assume it won't be enough for you until you decide that you want to invest in bitcoin.

Having understanding of bitcoin also is good but it is not a complete yardstick to measuring your readiness to invest. Bitcoin is too complicated to be understood within a limited time. So the best is knowing about bitcoin and owning a part even when you haven't know much about it. You can learn in the process instead of whiling away time learning what even those that have spent years in it have never understood.