Honestly in the right sense, Someone that doesn't have a Discretional income shouldn't have any business with Bitcoin investment because what's is needed to accumulate bitcoin is from his discretional income. Sometimes when he gets too excited investing in Bitcoin, he may start buying bitcoin outside of his Discretional income by using funds meant for his basics needs to buy Bitcoin regularly.
This is where good cashflow management practices and proper discipline take pre eminence. With good cashflow management practice, you'll carve out your expenses and separate it from your discretionary income and further divide your discretionary income into funds for Bitcoin accumulation and those for building backup funds. It's important to note that buying Bitcoin with funds that are not your discretionary funds can be described as being over aggressive and it's not a good Bitcoin investment practice and furthermore it's not sustainable in the long-run.