Yes, some people complain about style constraints from time to time, but the point is, that it's meant to be fairly straightforward and interchangeable (or at least easy to adapt).
"Semi-colour" - I think it was rather "semi-
contour" - meaning, that it's primarily used in places, where the actual black (used for contours, right angles and the like) wouldn't be used, but which are "almost-like-contours"/sharp-ish breaks in surface etc.
The adjustment of Lum-value by 20-40 was meant mostly for colors that are not too dark, as otherwise, of course, there is not enough value to adjust before going down to black. In these situations I adjust by smaller values, but still try to keep the intervals between them reasonably regular (especially between "big three" - highlight/basic/shadow, as these would be used for greatest percentage of painted area).
And last but not least, the matter that certainly is of greatest interest to You, which is commentary on Your new drawing. About this I can only say: Well done!
I would just suggest to try making wheels bit more rounded (on their outer contour, hubs are ok).
Also, although typically rubber tires are drawn with black outer contour, for some time myself (but also few other artists, I believe) tend to draw that outer contour in darkest shadow shade ("semi-contour"), as this isn't the most solid-est of shapes, after all (due to inherent elasticity of rubber and, where relevant, pressure in the tire). So I would suggest such option for Your consideration.
(This is an extension of style rule that has sails (and similar things) outlined in dark shade instead of black - You've already correctly applied this principle on tarpaulin.)
(mind that the line where rubber tire meets hub would still need to be drawn in black, because the hub is solid)