How to correctly determine proportions and perspective in different situations?

Question

3D

Question for professional 3D artists: how do you determine the proportions of an object in references, concepts and drawings when working in studios/freelance? What methods and tools of 3D packages do you use (rule of thirds, golden ratio, etc.)? I tried to learn according to the principle of this picture - https://drive.google.com/file/d/1-3KzyEGtqQ2ziUofA... , but either I'm a felt boot, or the picture doesn't work: how to apply this rule of thirds on a real 3D model in software ? I correctly understood that the truck on the right-top is used as a classic substrate ( that is, the blocking from below was made immediately taking into account the perspective), or the model was made according to the reference, and then the angle was set for correction? Also, do I understand correctly that the reference Bounging Box is always built by eye and it remains on the conscience of the artist and is pumped over time?