You can earn fixed income from bitcoin trading. That's by determining the target profit. As has been stated before, profits from trading can't be determined then from that we alone determine limits.
Even if you have your own target of profit, it will not guarantee you to reach the target in fixed time basis (daily, weekly, monthly). Fixed income from trading is almost impossible, you cant predict everything correctly on trading.
Determining target/limit is indeed great, it can be used to control your trading activity only (to avoid big loss or greediness after profit) but it wont help you to earn fixed/stable income from trading.
Yes, you set a target to trading, that is your work. The market does not care you can earning through trading like this target or not. Nothing guarantees your target will be reached because fixed income from a unstable job, it is impossible, IMO.