It all depends on your discretionary income and not your income mate. After you have received your monthly income, you are to take out money for your monthly expenses and basic needs. The leftover is your discretionary income, that's is the money that you are yo use part of it and invest in bitcoin.

You cannot buy a fixed unit of bitcoin 0.0000085 because bitcoin price fluctuates. If not, you will put your bitcoin investment at risk because you might invest beyond your discretionary income when the price of bitcoin pumps higher than what you bought. For example if you bought 0,0000085btc at $100 when bitcoin price was at $108k and now that bitcoin price is $117k, if you want to buy the exact quantity, the price will be above $100 which is more than what you budgeted for. At the end since $100 is your discretionary income, it will lead to you selling your bitcoin too early when your basic needs arises because you bought with part of the money for your important needs.

However, having a fixed amount in which you can use to be buying bitcoin weekly from your discretionary income is the best as long as it is an amount that wouldn't put pressure on you. That will enable you continue buying every week consistently and persistently for 4-10 years and above till you reach your bitcoin target. Even at this, there are some weeks that you wouldn't be able to buy with the same amount from your discretionary income due to increase in your monthly expenses. Some times, when your expenses is higher than the initial amount, it will affect your discretionary income which will make you reduce the amount that you will buy for that week and when your monthly needs and expenses his smaller compared to what it use to be, it will increase your discretionary income and you can also increase our DCA amount for that week.