7:19 PM, Saturday September 9th 2023
The second bullet point in the section that links to the mouthwash bottle demo addresses this concern:
We are not finding the accurate locations for each landmark. Rather, we are deciding where they will go. The reference image is a source of information, but how we ultimately use it is up to us to decide, so we do have the freedom to say "I'm going to go with 2/5ths instead of 19/50ths". Of course the more of this we use, the further we may drift from the actual reference, but for our purposes in this course that is fine. What matters is that the plan we create here, and the construction we ultimately create in 3D space match up and follow the same process.
Ultimately the accuracy with which we observe our reference image matters less than establishing a clear intent in what proportions we wish to employ. In the context of Lesson 5, if one were to decide to draw a dog with an unreasonably large head, but maintains those proportions consistently (despite them being wildly off from the reference image), it is entirely possible for them to build it up in a manner that feels solid and three dimensional. Thus, it would still meet the requirements of this course.
So, make your best estimate, and ensure that the decision you've made is something that you can refer back to to ensure that you don't change it halfway through your construction.