G13RL

Click on the handle with the left mouse button, hold it down and then press the right mouse button. Keep both buttons pressed while adjusting the handle.
When finished, release the left mouse button first, then the right.

Hi @DanM.,
I'd recommend checking out the Export Persona in Affinity Designer - as this allows you to export the image to a transparent supported format (ie PNG or TIFF) in multiple different sizes at once.
2022-10-14 10-41-39.mp4

In the above recording, after opening the image in Designer I navigate to the Export Persona and select the Slices Studio.
Here I can set the export to PNG, then add as many 'variants' of the export as required, setting the scale (ie 1x, 2x) or height/width (ie 200w or 600h) for each.
Then, you can select 'Export Slices' and choose a folder to export into - this will then resize and export the image in each size you have setup here.
I hope this helps

Je suppose que vous voulez parler de règle de trois.
Comme 100% est égal 360°, 90% est égal à (360/100)*90=324°
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I assume you mean rule of three.
As 100% equals 360°, 90% equals (360/100)*90=324°.
 

If it helps, someone (I don't remember who, unfortunately) kindly shared an .afassets file containing some perspectives on the forum.
To install it, go to the "Assets" tab of the studio, click on the hamburger to the right of the tab names and choose "Import Assets". On your disk, click on the imported file, "Open". A new category will be created in the "Assets" tab.

Perspective grid.afassets

@G13RL and @PixelPest, thank you both. Those were my first attempts, but neither can come close to accomplishing the hard edge to gradient part (and I actually have some of those effects still subtly on the different layers to "glue the mix" as we say in the audio industry), hence my searching for a more detailed method. I had that explanation in the original first paragraph but deleted it since I mentioned the layer effects mockup later on so as not to overexplain or have a massive post 😅 Guess I will add it back in haha. Always helpful to know where to add clarity!
And yes, that original layer effects mockup I did was with an outer shadow (along with the 3D and Emboss for making the edges less soft), and, while they all liked it, it did float off the page and felt too much like a drawing and not "this-is-intentional-white-on-white-carving" to me. But I figure, if you're gonna break all rules of art and fantasy, might as well do it as high quality as possible.
I may try this Blender tutorial I found and see if I can get it not to ruin the lighting. Thanks again!

Indeed, this is a clever way to proceed.

Thanks to G13RL and Bruce.
The following also seems to work:
Draw a rectangle around the intended PNG image.
Remove the fill and stroke from the rectangle.
Select the intended image and rectangle and export both as a PNG (Area: Selection Only).
Use the exported PNG for the brush.
See below...

You only need the area with the green frame.

Just draw an ellipse and then use the Export Person to export a slice which has space around the ellipse.

Thanks so much to everybody who replied, you all solved my maze problem. Your solution is exactly what I need and I would never have found that myself.
I do appreciate your help so much.
Chris
 

I can only speak English* and I sometimes... often find it difficult. I am continually checking the dictionary for definitions of words I have used for 50+ years.
* la Francophonie has a restraining order preventing me from even thinking about speaking French. Something about my pronunciation and grammar being criminal.

Personally, I use DeepL or Google Translate, especially for answers. I am French and when I realised (in high school, in the sixties) that my English pronunciation was catastrophic, I gradually gave up.

The proper name is the Context Toolbar. View > Show Context Toolbar.

Short explanatory clip.

Sorry, I did not make myself clear.
When you are working with the Pen Tool, its contextual bar appears, and this is where you have access to "Add New Curve To Selected Curves Object".

Here are a couple of styles, One open ended and one closed. You will have to apply them to the strokes after you draw them. They use the appearance panel to make two strokes.
experment.afstyles

Short explanatory clip.

I drew the black line (square cap), then, in the "Appearance" tab, I added a grey stroke a little less thick than the black, also square caps. 
I don't do anything special to avoid overlapping, it just happens by itself
The overshoots you mention only appear at certain zoom levels and are not present when printed.
See outline view mode attached.

Well, my sincere thanks to all of you for your help.  That definitely cleared up the problem.  I never would have figured it out on my own.  Apparently, while working in a text box, somehow I turned on the transparency tool, and that's how it connected with the text.  Who'da thunk?  Whew!  Next time I'll know better.  I'm very grateful.  I was afraid I had been condemned to having to deal with this forever.
In the future, I will upload the appropriate file -- I had no idea that was available.  See?  I'm learning.
By the way, Old Bruce, we have two things in common: I'm old and my name is Bruce.  Now, if you skydive, we'd be even closer.  How 'bout that!
Thank you to everyone who helped.  I am in your debt.  And yes, Santa has been notified.

Thanks for the feedback, it is because there is no shape builder tool, as mentioned @Alfred, that we are forced to use a workaround.

Thanks for the feedback, it is because there is no shape builder tool, as mentioned @Alfred, that we are forced to use a workaround.

If you want to get something like my example, select the set, "Convert to Curves", do a Boolean operation "Xor".
Then take "Node Tool", hold down the "Ctrl/Cmd" key and click on the curve segments you want to delete. 

If you want to get something like my example, select the set, "Convert to Curves", do a Boolean operation "Xor".
Then take "Node Tool", hold down the "Ctrl/Cmd" key and click on the curve segments you want to delete. 

The donut indicates that inner radius and hole radius of the cog are equal. Then Geometry > Separate Curves would indeed result in single donut segments.

@Ehrie, if "Inner Radius" is equal to "Hole Radius", "Separate Curves" will do the job, as @NotMyFault indicated.

@Ehrie, if "Inner Radius" is equal to "Hole Radius", "Separate Curves" will do the job, as @NotMyFault indicated.