this question has been asked a lot of time and at this point I will choose to agree that we not being truthful to ourselves. We all know how much we are earning and we understand how much we can afford to use for an investment that will still give us the leverage of living comfortably.

Understand that when it comes to the amount you should use in making any investment, it shouldn't seem like their is anything too special because you are investing in bitcoin, take it that you are buying a known asset like land that you know that within two years, it value will appreciate. How much of your savings would you comfortably put into purchasing that land? You know that the price of that land you're buying won't just appreciate like that and what you need do is to spend an amount that wont keep you hungry after making the purchase.

It's as simple as that when you intend buying or holding your bitcoin, buy what you financial strength can carry.