When all corners of the room are 90°, the diagonals are of equal length.
So-called cross measurement.

If the existing corners are not right angles, you should start from the parallel wall and place the new one equidistant from it at both ends.
 
Can't you make a simple sketch of how it looks, then it will be much easier to give a good explanation....

It's hard to really understand what you mean otherwise.
 
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