Alright, so how I did this was pretty straightforward with Illustrator. It's how I do, but you might have another preference. I don't have affinity designer, sadly, just an ancient-ass version of CS.
I had my art auto-trace vectorized as grayscale, so everything (at least if you
start with a file in CMYK) was a percentage of K (black). Then I go each tone one by one and select every piece of geometry that had that color. And then used the Pathfinder tool to unite them into a single object (well, group at least…) Then I named those groups based on the amount of black ink:
I then took those numbers out of 100, and made them the corresponding Tint percentage of the RDG_WHITE spot swatch color. BUT I did do some refining, because it wasn't quite what I wanted. I used a QaD trick to simulate what it would look like by taking a screenshot of the RDG_WHITE artwork, popped it into Photoshop, inverted that, and used the levels to set the white point of the image to what I knew was going to be the "whitest" part of the design:
After some adjustments, I had it where I wanted. Of course, RGB =/= CMYK, even with grayscale, and a given print will typically be darker than what you think.
Here's an incomplete breakdown of the tint percentages used. AS ALWAYS YMMV.
I noticed the dithering on the white ink percentage is not that dissimilar from the classic Photoshop diffusion dither pattern used for monochrome image conversion (pictured):
And of course, per the standard file prep, there should be NOTHING except RDG_WHITE in your designs. I put a FFFFFF white background so it wouldn't look so awful.
