Good Day, newbie,
I am a fresh boarder as well. I would like to try and make a suggestion and the senior members and masters will hopefully input or correct me. I would be confident to say that zi wei , I think is more square than round. I did research translating my broken Chinese and rectifying it in understanding charts and what I understand is that the rectangular chart is actually circular, so when you picture the zi wei chart imagine that the houses or spaces are arranged like the ba gua.
When you redraw the entire zi wei chart and fit it in a circle with the same order and arrangement as how you see it online or however you did the computation, wherever the opposite location falls that is what I think is the opposite house. After this you can actually visualize the circular arrangement in the rectangular arrangement of the chart.
Senior members please correct me and rebuke me if necessary as I am also new to this but am beginning to understand layouts and charts and how everything ties up to taoist principles.
So with your question, whatever the opposite house is from the current or referential house is (diagonal line from reference point) that is the opposite house or court. or if you like you draw a circle, so if a zi wei has 12 houses, however the numberings are , the opposite house of any house always falls diagonally from the referential or indicated house. Say I start with house 1 in the top middle of a divided circle the opposite of that falls directly underneath that meaning the opposite house is the house underneath house 1 at the bottom of the circle. I am too verbose but I think that's how you find opposites, pretty much like a circle in geometry. please comment senior and advanced members on my input.