In order to maximise or optimise your DHX mining, you will need to have a sufficient amount of mPower for each DHX that you bond (i.e. earn the full 1/70th DHX of your bonded DHX per day).
You can use the DHX Mining Simulator in the DataDash app to determine the current amount of mPower (and MXC) needed to maximise your daily DHX earnings, in different scenarios. If you don't have the full mPower needed per DHX, then you will just mine proportionally less DHX.