This is my Stoopid Test Cube, there are many like it and this one is nothing special, just a block with a square hole. Specifically it is 25mm square, 5mm high and has a 10mm square hole in the center. No science was involved in it's design, and I didn't put much thinking into it, just wanted something quick to print that would do the job.

Also, just to note, I'm no expect on 3D printing, but I do have a couple years of practice (which again does not make me an expert). So please let me know where I messed up in the comments, and I may amend my ways. I use this thing to calibrate flow first, and then horizontal expansion only when necessary. When I get it close, I stop wasting filament on these, since Perfection is not a Pokemon that is worth chasing in my 3D printer world, Pika-closenufa-chu works pretty well.

To print this for calibration of flow, I use the following settings in Cura (in addition to whatever is recommended for the filament of course):

• Quality > Layer Height 0.2mm (for a 0.4mm nozzle, adjust as needed if using a larger or smaller nozzle)
• Shell > Wall Line Count 2
• Shell > Top Layers 0
• Infill > Infill Density 0
• Material > Flow 100

Additionally, take note of what Shell > Line Width is set for (it may be the same as the nozzle diameter).

Once it is printed, I measure the wall thickness from the middle of each wall, measuring about a mm or two up from the base, keeping the pressure about the same on the calipers for each measurement, and then average the measurements for all the walls to get the value to use for the flow calculation. If any walls are way out vs the others I will not use that wall measurement for the calculation.

To calculate flow:

V = average thickness of the walls
F = current flow setting (start with 100%, or 100)
W = wall line width, either automatically determined by Cura or set in the slicer
n = number of walls (wall line count) used in the slicer (2)

F x ( (W * n ) / V ) = new Flow Value

So for example, using two 0.4mm walls, and starting with 100% flow, with a V = 0.9mm (which is the average measured width of the test cube walls), I would get this:

100 x ( (0.4mm * 2) / 0.9mm ) = 88.9 or 89% as the new flow value

To be honest though, I don't always use the calculated flow value. When I get the calculated value I will print one more cube with the new flow value, and if it looks less than good or shows issues with adhesion or something like that, I will adjust the flow based on what I use for similar materials (may be up or down which is why I keep track of them). The calculated flow value is a a good place to start at least, and often spot on, but I don't think of it as being some sort of rule that cannot be broken. If printing parts that don't need to fit together or fit other parts, then flow seems to be not as important anyway, as long as it's in the ballpark things usually look OK.

So now you may be asking, why the hole? I use that for horizontal expansion calibration, but only if it is really needed. To do that I measure the outside dimensions of the cube (outer wall to outer wall) and then compare that to the actual size it should be (25mm), and then check the same for the hole (which should be 10mm). If say, the hole is only 9.8mm on average, and the outside average measurement was 25.2mm, then I would bump horizontal expansion by -0.1mm (Shell > Horizontal Expansion). That will tell Cura to contract the sides by 0.1mm, bringing the outer walls in, and making the hole walls further apart. In practice however, it seems to be more a balance, since horizontal expansion can do odd things to prints. Best to watch the Chep videos below to get an understanding of how horizontal expansion works. In Cura 4.6.1 and higher there is a feature to only expand or contract holes which is really handy too (Shell > Hole Horizontal Expansion), it is explained in the second link to a Chep video below. Note I can't seem to remove these youtube videos from the pics preview, seems Thingiverse is automatically adding them to the pics, unless I remove the links. If you know how to post youtube links that don't generate a preview in the pics, please let me know.

Chep video explaining Horizontal Expansion:
https://www.youtube.com/watch?v=-jsBI3OeUJQ

And a newer Chep video with info on Cura 4.6.1 and higher's "Hole Horizontal Expansion" feature:
https://youtu.be/UUelLZvDelU?t=90

So that's all I usually do to tune my settings better for a new filament (aside from possibly retraction, temps and fan for stringy materials). Just note that steps per mm should be done prior to messing with any of this stuff since that is printer specific and all the flow and horizontal expansion stuff is material specific.

I also find it helpful to keep track of these settings for each filament type I buy in a Google Sheets Doc, which makes it simple to get in the ballpark quickly between different brands and types. Good luck!