From experience I've observed that waiting explicitly for what is left over after taking care of all expenses before investing into Bitcoin is an evidence of improper planning. You would end up servicing even less important things and at the end nothing might really be leftover to invest into Bitcoin.
It's important you take into consideration your likely expenses for the month, calculate it by yourself with acceptable tolerance to entertain price spikes, dedicate specific funds to it and plan out what percentage of the remaining funds would go into your accumulation of Bitcoin, backup funds and still leave some floating cash to serve as miscellaneous in case your expenses planning experiences a shortage.
You might get it wrong for the first month, however from the second month you'll start getting better at planning out your expenses and using funds judiciously to attend to it without being as wasteful as you can be when you don't plan for it and then you can keep getting better at both your cashflow management and may even consider increasing your level of aggressiveness in your periodic purchases.
I think it is still better to wait until you are able to figure out your discretionary income which is your leftover after you have settled all important needs before you can start accumulating Bitcoin from your discretionary income, for it is wrong when you use money which is not your leftover income to invest in Bitcoin , investing from your discretionary income will enable you HODL your Bitcoin for long and it will also make you not to sell out when it's not yet your target time to sell because you are not using money meant for family needs or up keep to invest into Bitcoin because when you do so you will definitely sell out your Bitcoin even at lost when more needs may arise so no matter what it is important we have our discretionary income before coming up into investing in bitcoin.
Well if one invest in Bitcoin with an discretionary income it allows for an long term preventive reduction of selling permanently, some investors failed to understand this when investing you need to prioritize needs over wants to ensure an essential extra expensive and are covered before you invest in bitcoin, and they need to understand the discipline approach to invest in Bitcoin, minimizing financial stress and maximizing your potential returns.
Don't make it look confusing on what you are trying to express, what you are trying to say is that as a Bitcoin investor, in other to be successful, you need to be financially smart and also have a good management skills so that as your paycheck comes for those that are working, you make all your planning according to your budget, which is money for you basic needs and upkeep, the amount of money allocated weekly or monthly for buying of Bitcoin, and you also allocate some money for building up your emergency funds and your back up fund's if possible.
So what am trying to say is that if their is proper money management from a Bitcoin investor that knows all that it entails to buy Bitcoin and what are the measures that needs to be in place to be able to hold, their is absolutely nothing stopping that person from being successful in his holdings, if he can be able hold for a very long period of time.