Associate Concept Artist at Riot Games.
You can always paint yourself a form ball in the corner. Figure your light source out exactly, paint a plain ball in the corner and then you have all your values arranged and ready for sampling. Just look at the area of the image you're confused about, map it's angle to the light source according to your simple ball and you've got your base value. If local value fucks it up, just tint the ball to your local value momentarily or with an adjustment layer and resample. Simple old trick that still works every time.Of course, if it's the form itself that is confusing, there are other tricks. I fall back on Carl Dobsky trick for the doofy amateur artist #36: the mountain climber. Picture your light source as the sun, and yourself as a tiny mountain climber hiking over the planes of the form in question. Gimpy, but works for me.Sorry to babble all over your blog :P