Translation:The calculation of this square's angles is already correct.
Not much to calculate in the angles of a square. The corners are 90 degrees and the diagonals are 45 degrees, or else it is not a square.
I would translate the meaning this way: "The calculation of the angles of this square is correct." Also, the word "already" is redundant in the English sentence.
It's not redundant. It adds emphasis, you just have to make up a context.
Student: "I'm trying to figure out the answer but I don't think I have this calculation right" Teacher: "Well the calculation of the angles is already correct, so just move on to the next question"