That's just the point! Can you not see how this question tacitly concedes the point?
Your default mindset is that justice is determined as if it could be something other than what it is. God did not sit down one evening and decide what justice was going to be. He did not determine what it means to be righteous or what it means to love someone. These words are used to convey certain concepts and it was the words that were created not the concepts that they are tied too.
In other words, a rose by any other name is still a rose. No one, not even God, had this word lying around waiting for a concept to tie it too. There is a plant that produces a particular kind of flower and whether we call it a rose or a fart, it doesn't change what it s.
The same goes for right and wrong! There are concepts that those words are tied too. If you help your neighbor who is in need, we call that "good". If you do unto a criminal as he did to his victim, we call that justice but those things are not righteous because we use the term "righteous" to identify it, they are what they are whether we call it "righteousness" or "califactal" or "purpline" or whatever other made up word you want to use.
God did not decide what is and is not righteous and if He had then there is no sense at all in which you could meaningfully call God righteous or just! Besides that, to say that God created or determine justice is to say that God created Himself!
Do you believe that God is just? Do you affirm that God is righteous? Why? Why is God righteous? Is it simply because He said so or is it because of how God conducts Himself in thought, word and deed?
Deuteronomy 32:4 He is the Rock,
His work is perfect; For all His ways
are justice, A God of truth and without injustice; Righteous and upright
is He.
John 5:31 “If I bear witness of Myself, My witness is not true.
36b for the works which the Father has given Me to finish—
the very works that I do—bear witness of Me
Clete