I’m editing my pictures on Photoshop. As a rule of thumb, what % should I increase the brightness/contrast to so the pictures accurately portray the original pictures when printed.
there is no rule for all designs. You just need to increase brightness if the design is very dark.
Increasing contrast is most time a good idea when the shades in an image are all very similar or close to each other. If the image already has high contrasts you dont need to icrease them.
Just keep in mind, that an image on a monitore shines by itself - the display emits the light itself.
But on a printed product - the product itself doesnt shine by itself. It just reflects the light from the environment and therefore never can be this brilliant as it is on screen.
I hope this helps.