For your life total to become your starting life total (normally 20), you gain or lose the appropriate amount of life. For example, if your life total is 4 when Oketra’s Last Mercy resolves, it will cause you to gain 16 life; alternatively, if your life total is 25 when it resolves, it will cause you to lose 5 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.