The only parts of these we don't agree with are how to determine the bra frame size. One nice thing about being able to make your own bras is that you can accommodate a figure where one breast is larger than the other and you can make your own swimsuits or other apparel that you'd like to have built-in shaping and support. I can't prove this, but I believe that most sizing systems were devised by marketing departments solely for the purpose of obtaining a competitive advantage, NOT to simplify shopping, sizing, or fitting. You'll feel better. When discussing cup size, we have to consider projection in addition to width and depth.
Projection is defined as the distance the cup extends from the rib cage.
Viewed from the front, bra cups are U-shaped. Width is the distance from one side of the U to the other. In answer to your question, the projection of a given size cup is typically the same for all bras from the same manufacturer. Since all manufacturers use different patterns, some E cups will have more or less projection than others and that's why a 38E from Just My Size may fit differently than one from Olga, for example. Cup width and depth, however generally increase with frame size and these too vary not only from maker to maker, but also from style to style for the same maker.