I am a technical guy -> artist.
So, when I say computing highlights.... well, I guess I do mean like what you would do on a computer. Here's the window (light sources), balloons are shaped like this, compute reflections...
These are what go through my mind. I don't think they go through other artist's minds.

I have seen some water color artists add highlights by washing. Specifically soap and water and scrub an area. But, that is probably more in the mistake correcting arena, that you wouldn't do if you planned your hightlights out before the brush got to the paper.

And then there is always cheating. Using white paint. Which I find almost essential for things like the bright spot on the eye things.

This painting came out very nice. Have you thoughts on framing it?