You need a discretionary income to reach your goal. If you set a budget to reach your investment goal, then maybe, you can step away from your investment. For example, if you set a budget that you will buy using the DCA method for the next 10 years, how will you cope if you face a financial disaster?
Dont make things more complicated. I believe there is no one here who dont know the essence of having a discretionary income. And yeah, your description of it may seem to be unclear. For better understanding, any money use to start an investment or a business is called a capital. The total discretionary income, after completing your goal can be said to be the capital invested. You can choose to put the capital once if you have it, that is if you are using lump sum. While DCA actually means spreading it across several periods.