TAZ 6 Frame squaring

I have an early TAZ 6 that was affected by the issue where the printers frame might have gotten out of square while shipping. I have tried to square the frame according to these instructions: https://ohai.lulzbot.com/project/squaring-taz-6-frame/service-bulletins/ but I can’t get the diagonal measurement to be equal. I always get 74.5 and 75.5cm if I put the extrusions flush on the bottom. Just by looking at the frame visually it looks a bit out of square.

I tried to measure the angle between the x and z-axis smooth rods with a digital angle gauge and it shows that I’m 0.4° off. I don’t think that is much to worry about but now I got a small print that was about 3.2° out of square for some reason. On the print the angle between z and x-axis |_ was 87°. The smooth rods |_ were 89.6°. Measuring the aluminium extrusions I get the same ≈0.5° off.

y-z-plane was bang on within 0.1°.

As the model I printed was quite small, it was quite hard to measure the angle accurately. I try to print a bit larger box and measure that as well.

I printed a 50mm cube in vase mode. It clearly shows the 0.5-1° misalignment. The 3° I got earlier might have been just a measurement error.
How should I go about squaring the frame properly.
Here is a scan showing the xz-plane:

yz- and xy-planes are fine.

I needed to revise my elementary school geometry. As the layers stayed perfectly level, the frame was a perfect parallelogram.
I did the frame squaring once more but now I had a friend tweaking the frame while I tightened the screws. I got it to measure 748mm all around. Hopefully this issue is now fixed. I’m printing the test cube once more and I’m quite confident I’m going to be pleased with it.