Prusa i2 ensuring Z Axis is Orthagonal?

This is something I've fought with since I put this together years ago. I've been through multiple tear downs and rebuilds and never really have found a solid way to get it right.
I want to do it one last time before I print parts to replace this machine.

I've rebuilt the frame, Just the triangles and cross rods. Ensured it's square and tread locked it. This is really the last time I'm doing it. All the other components that affect the alignment of the axis, I've not thread locked.

I've got X and Y perfectly orthogonal to each other.
I've printed a single layer square slightly smaller than the size of the bed and measured diagonals, it's square

But I notice that when I print something that's towards the upper limit of the printer, say 80-90mm tall, that there's a very gradual lean. I never really printed parts that tall so I never really bothered to fix it. Now I want to for my replacement machine.

Getting X and Y were easy, I did #7 from the RepRapPro Tricolor Mendel Build Guide here:
https://reprappro.com/documentation/mendel-tricolour/frame-assembly/

And I did do #6 for the Z axis, but I found the string and measure to have to much room for error.

Anyone have any tips on making sure the Z axis is orthogonal?

Only thing I can think of is to print something like a cube as tall as I can on each side of the print bead as close to the smooth rods as I can and measure how much they're off. Or do a square that's like 10x100x100 (x,y,z), again on both edges of the bed near the smmoth rods and measure diagonals on them in the Z direction.

Use this: calibration cube

You check the XZ and YZ planes the same way you check XY- print a square and measure diagonals. This object does just that and comes with a spreadsheet that calculates the error based on the diagonal measures. It then tells you how to adjust the guide rail positions to correct the errors.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Coming back to this, calibrating my extrusion and axis steps per mm to get accurately sized parts before I print this calibration part. Using PETG, printed a 100x100x10mm part that when measured gives me as follows.

Looking at my Prusa i2 from the front. Y Axis being the table that moves front to back, X being the carriage that moves left to right.

Left: 100.1
Right: 100:10
Front: 100.24
Back: 100.19

Could it be because something isn't quite orthogonal that the left and right side are different from each other, and further still, different from the front and back?
Should I just print out the part and use the spreadsheet?

So looking to ditch this machine after I get it to print the best parts it can for it's replacement.

Finally took the time to get this done once and for all.

First I bought a Speed Square, generally used for Construction Framing. But I liked that it had a wider base on one side. I could sit it on end and it wouldn't rock.
I bought this one specifically because even though it's cast aluminum, it has CNC'd edges

Johnny Square

Then I tested it by using an edge that I know is straight, put it on the edge and drew a line. Then I flipped it over and drew another line. I then measured the distance apart at each end and they were exactly the same, ensuring it's square.

I then took the bed off my machine and measured the distance from the Y Rods to the table at both ends. I had to file a couple of my feet to make them all the same.

After that, I put the glass from my Print Bed across the Y Rods and put the Square up against my Z Rods in the X direction.



I did the same thing on the other side, I adjusted as necessary, then I measured the distance between the tops and bottoms with my digital calipers, slight adjustment made them exactly the same.

I then constructed a wooden table out of some Laminated MDF shelf (flattest material I could think of that's not Cast Tooling Plate), so I could measure them in the Y direction.



Then I measured and adjusted as needed. Again same on both sides.



I then put on the X Axis, and used it as a reference to make one of the Y Rods orthogonal.



I made the other Y Rod parallel to that one.

I also converted to a 3 point level on my Print Bed.

Took the PCB heater off and drilled a hole at the center of one of the edges. I had to cut the traces to ensure the screw, washer, and spring wouldn't touch any electricity. I also lost the ability to hook up a LED, but I wasn't using it anyway. The result is much easier to level.



My first print was the 40x10 Single Wall Calibration part (I also switched filament) and the walls are perfectly aligned, as are the corners. The best I've ever seen with this machine.