Hm
Depends what effect subtle or otherwise you are going for.

Trick I use is...application of a non-existing light source.
For example ...pick (at 1st ...1 directed light source...later playing with multiples becomes easier.) imagine said light source static and stationary ...imagine how said light source will interact with your imagined subject in your head....and apply. That way you are closer to realism and there aren't confusing or impossibly angled light or shadows.
A more direct and visible answer you can see...
take an apple.
take a low or high watt light that has a shade...I find for ease of use ...a multi-angle armed light that can be easily moved... adjusted for simple affirmation of my internal judgement on the physics of light ...when I'm in doubt.
and take a sheet of white paper.
and a table
and a chair...
In a dark room set up your light...put the apple on the table...
shine the light on the apple...
set up your white sheet of paper...(considering where you are sitting on said chair) behind the apple ...other coloured paper helps as well....

Looking at the illuminated apple ..which has curves rather than edges...you can see...the apple at it's least illuminated blends seamlessly with the darkness...
On it's average...the apple has the appearance of a distinctive edge...
and at the angle where the light is shining brightest...the eye loses the ability to perceive the edge again.

That is roughly the dynamics....
But it really does depend on what you are trying to achieve...
there are a couple styles of illustrative work that rely upon a ...umm..."containment" ...kinda line around the edge of the work.

So in essence it is subjective ...for you the renderer ....it depends what effect you wish to accomplish on your viewer.

I think you did a decent job.

It does what is intended...you have wrought his face well enough for anyone to see it to immediately know who he is...

As well.... you immediately (with the content)... evoke a feeling in the viewer....(a very powerful thing an artist can do ...not easily accomplished ...nor easily maintained...)..from my humble opinion...mission accomplished.