🚀 go-pugleaf

No. Do not forget your color settings in Photoshop compensate for dot gain. Adjusting any further than custom curve percentages, such as an overall CMYK shadow adjustment down to 85% is overkill and will weaken color hue, contrast, and brightness. If your scans are coming in at around 300% total ink density, then leave them alone.

Now, who's to say that in some cases you'll want to boost Cyan shadow in order to get bluer mountains, darker sky, or deeper black? Each image is different and should be treated individually. Your friend has given you guidelines on where percentages should fall and I think you've got a good understanding of those numbers. No need to over-do it.