You may decide to buy a commercial gray scale. The grays are ten percent gradations.
Another possibility is to use your computer to change your painting to gray scale to get ideas for continuing or to confirm that you have a variety of values in your painting. Here is an example of checking values by changing a color painting to gray scale.
From color to gray scale
It helps to know the value of colors as they relate to the gray scale.
Here is the same photo but changed to gray scale with the computer. You can see that the bit of neck surrounded by the 10 percent gray keyhole is even lighter than 10 percent gray. The third value down, 30 percent gray is close to the bit of scarf in the keyhole. That orange is close to 30 percent gray.
Using value contrast is one key to making a painting that attracts the viewer.