This article may help a little.
http://www.imaging-resource.com/TIPS/PRINT1/PRINT1A.HTM
And there are several articles on resizing in the CinC Tutorials; like this one
https://www.cambridgeincolour.com/tu...ze-for-web.htm
Your image here is actually 546 pixels wide. How did you upload? Some methods get auto resized downwards.
So if you have gone from 2848 pixels wide to 546 there is likely to be quite a bit of softening unless you re sharpened after resizing. I normally use Unsharp Mask around 60% and 0.4 pixels after a substantial downsize. Other people have slightly different settings but the principle remains the same.
Some sources recommend using an auto resizing sharpen like Bicubic Sharpener but I found the results were often too little or too much. So I prefer to do it manually.
Your mathematics only really matter when resizing for a specific print size. Otherwise it is just a case of working in actual dimensions like ins/mm or in pixels; depending on the purpose of resizing.