How to add 15% more black to a graphic in illustrator or photoshop?

Client is asking me to add specifically this much percentage of black into a graphic. How do you do that?How to add 15% more black to a graphic in illustrator or photoshop?
That could be referred to dot gain for printing separation. Take a look at the links below.How to add 15% more black to a graphic in illustrator or photoshop?
black layer will work, but i recommend going into levels or curves and turn down the brightness. you get better contrast and range this way so the picture is more alive, unless of course you want it dull.
Perhaps create a black layer over the graphic and put the layer's opacity to 85% .
totally beats me, and i am a learning artist. it might help to list the programs and tools you are working with