I think the best way for someone without a fixed income to invest in Bitcoin is getting the average of what they use to earn or make and then take some cent from the average and be investing with it.
This is surely talking about investing with what you can afford to lose, but the way you explain it makes it look somehow different. We have said it repeatedly that guys should figure out how to invest with the little they can afford to lose, as this will help to maintain a good investment strategy to avoid selling your Bitcoin too quickly. if it happens that you are investing aggressively without a good reserved fund you will fuck up down the road due to bad financial management.
With this, one don't have to struggle on the amount of money to be used for investment as someone whose income is not fixed instead of investing with random amount because you might definitely make a mistake and so whether fixed income or not it doesn't matter, what matters is the ability for you to do these mentioned above.
The question is that are you investing from your salary or from your discretion fund. Yeah I quit agree with you that a person might have an epileptic or an Iregular source of income but if he has a sizable reserved fund then he may maintain a fixed amount of accumulation since a reserved fund is a backup fund to help in Bitcoin accumulation in the absence of income.