Presumptively if one is reasonably establishing his basic expenses, then there is no discretion in regards to what is discretionary income, since whatever income is left over after basic expenses is discretionary income.

Sure some folks mis-assign some of their expenses as basic, when those expenses are in fact discretionary.... and I am not even proclaiming that the line is clear in regards to which expenses are basic and which expenses are discretionary... yet the idea is that if the expenses are basic, then we might not have much of a choice, such as our rent our our car payments or our utilities or our transportation costs, even though we might know certain ways that we could save money, even within our basic expenses.. ye t some of the basic expenses would take more time to cut than others, and some of the basic expenses are actually better not to cut so that we can improve our health and well-being, since it might be better that we live in a comfortable and safe place that is close to where we work, rather than living in a place that might be much less expensive, but maybe not safe or close to our work or even comfortable in terms of size and/or other modern features that might help us to be more productive.  We can buy quality food that does not cost much or low quality food, and sure there are various combinations in regards to the kind of food that we are able to buy , and we can attempt to plan around our food selection choices.

So yeah, we know that discretionary income can be increased by increasing income and/or cutting expenses, and once we establish our discretionary income we can do whatever we like with such discretionary income.. that is why it is called discretionary income, yet the three categories in which we can do whatever we like fit into 1) consume, 2) save and/or 3) invest.

The percentage of cost per expenses and/or what is left over (discretionary income) might be a little bit beyond our control in great ways, and it might take some time to figure out ways to increase it, in the event we wanted to be able to put more aside for investing into bitcoin or maybe to make sure that our back up funds are sufficiently large in order to give us a cash cushion in case we make mistakes or in case our expenses as compared with our income.

I am responding to these posts in the first place, since it seems that some members seem to misunderstand the order of choices, since we do not just choose our discretionary income, even though we choose the kind of work that we do and the kinds of things that we buy, yet some of those income and expense matters are not very flexible, so we should still be thinking about choices within discretionary income once it is established each month (or each pay period) rather than presuming that we choose the discretionary income first, which is a wrong way of thinking about discretionary income.. or even discretionary funds that we might have available to us. 

Discretionary income is a flow of income coming to us after our expenses, and discretionary funds can be money taken from any place or just money that we have available that is available for us to do whatever we like in terms of consume, save or invest, since it does not need to be used to cover basic living expenses.