I have heard before of using the hypotenuse to form a right angle and then a parallel line but using true north instead of magnetic north to line up your building is impressive. Thanks for demonstrating it so clearly.
When I do fieldwork, I often use the built in compass app on my iPhone. It could display both true north and magnetic north and it has helped me out a bunch of times. Do you think that's accurate enough compared to your method? I find QGIS is a nice open source GIS software for DIYers looking to do some survey/map work, I think it might be useful for you if you plan to expand your farm, as you can measure things directly using UTM coordinates and compensate for shadows, sunlight angles, etc.
Since your have set one point of the second long side already, how about using the same 3-4-5 rule to the other end of the long side(1), so you can get the second point of the long side 2. With two points of the second long side decided, there you go: you got the second long side, which is for sure in parallel with the long side 1. Would you think this way suggested would be faster than you current way of repeating 3-4-5 rule for extending? Thanks so much. Best. Actually, never mind. The iron bar is only 8 meter long, so it has to go by extending, you are right, Cai Cai. Thanks Cai Cai and Guo Guo for showing us the life of peace and nature that you are building. Thank you so much.