The best way to create your watermark element in your design file is to use a vector logo or shape comprising of single colours.
The look that you want to achieve is normally about 15% of a solid colour.
Here's a visual example that compares the percentage you see on screen VS the true output on print.
As you can see, it prints significantly darker from what you see on screen. This percentage of colour from a solid 100% typically prints very light to allow writing without the interference of the logo.
This particular colour of green will require a much lighter ink density to appear faint and subtle on print. However, other colours may not require such a low-level ink opacity to achieve the same output results.
For the best and most accurate results, we highly recommend that you request a hard copy proof when placing your order. It may help you see the results on paper before running a large print run.
The end result may vary from hard copy proofs and the final run, however, over time you may adjust these settings as they get closer to your desired look.
This graph is not a guaranteed graph but this is a typical visual representation of the inversed relationship between the darkness of the colour and opacity you set in percentage when creating your design.