I haven’t seen Scott Robertson’s videos (yet!) but was curious and saw this on a Concept Art forum discussion. It isn’t clear to me not having seen the video but maybe it will help:
“First thing to do is to decide what your light value is, and what your shadow value is. The easiest way to go is to make the light 1, and the dark 6. This is the half-way-to-black relationship: The way it works is, using an 11 value scale(0 to 10; 0 = white) whatever value you chose for the light it’s shadow will be halfway to black from it, so if you pick 1, you’ll be left with … 2 3 4 5 6 7 8 9 10, and 6 falls in the middle. You could chose any ratio, could be one third to black or whatever, all that matters is that you’re consistent.
Next choose the direction of your light source and represent it with a sphere. Lighting the sphere is basically dividing it by it’s circular cross sections, which requires perspective. You can see I used 6 values on the example. I did this quickly and the values used are arbitrary but I think it gets across the point. You should also do an actual rendering of a ball, or an egg from life, to get a more believable turn to your form.
Refering to this while you draw you can see that whatever planes face towards us will be a value 3, the planes facing all the way to the right will be a value 5, etc.”