Lithophane Gradient Test (Brightness Test)

You can use this test pattern to determine the correct thickness of material you need for different brightness values.

If you don't want to print a 100x100 test part, then scale the XY, but do not scale the Z.

If your material needs to be thicker than 3mm to be fully dark, then just scale the test model in the Z axis by 2X, and multiply the resulting values accordingly.

If you are looking for a Lithophane conversion tool:
http://3dp.rocks/lithophane/

Good Luck!!!