There are 3 questions I try to ask and answer when painting a value (shade of light or dark)
1) Where is this plane/surface I am now painting in relation to the light source/s and reflected light?
Is it directly facing the light source? Directly facing away from the light source? Or somewhere in between.
2) What generally happens to the shade/value of a surface in that position relative to the light source/s? How does light generally act on a surface in that position relative to the source? Is it in shadow, half-light, no light..
To help answer this question there are general rules to how light acts on a surface as it turns away from the light source including:
Highlight, Center Light, Midtone, Terminator, Core of Shadow, Reflected light (in shadow) Occlusion shadow, and Cast shadow – see this great tutorial
3) How much?- Or now that we have clarified where this surface is relative to the light source and we know the general rule for a surface in this position – now we can ask – how much?
To what extent does it get darker or lighter relative to the planes or surfaces in the rest of this painting?
I have found this last question to be the most difficult part for me to answer.
For example in the above Jet Plane picture let’s analyze the back vertical fin facing us:
Where: The back vertical fin of the jet plane is facing away from the light source – which is above and behind the plane.
What happens to a surface in that position? It is in the form shadow with a little cast shadow on top from the fin overhang - it will be darker than surfaces facing direct light.
On the other hand it is vertical, exposed to reflected light from the upper part of the jets body and surrounding clouds and mist/atmosphere maybe even earth, ocean, or mountains below- so it will be lighter than the dark cast shadows that have little reflected light and certainly much lighter than the occlusion shadows (cracks or squishy spots) that have no light penetrating.
How Much? – The fin must be darker than the direct light surfaces, and lighter than the cast shadows, but how much in the middle is it? Where on a scale of values from the darkest value in the picture to the lightest value would it go?
I have a hard time answering that question.
One helpful general rule I have heard is that “nothing in shadow (out of the direct light) is ever as light as even the darkest part of anything in direct light” or “The whole shadow family is darker than the light family” - even when reflected light is taken into account.
So now we know that every part of the fin must be darker than any part of the plane facing direct light- no matter what value scale we use. If you squint at the picture you can see that.
Other than that helpful hint I just wing it – pun intended
The only other thing I can think of is whatever value you pick for that angled surface - somewhere between direct light and cast shadow- be consistent in the rest of the picture for planes/surfaces in similar positions.