Zygarde-Complete bulk analysis

When EVing for general bulk on Zygarde, damage calculations concerning general bulk must be considered both in the context of Zygarde-50% and Zygarde-Complete. To start, let's look at HP:Def ratio examples with Tapu Lele's Moonblast as an arbitrary attack against both Zygarde and Zygarde-Complete with 252 HP / 4 Sp. Def and 4 HP / 252 Sp. Def.

252+ SpA Tapu Lele Moonblast vs. 252 HP / 4 SpD Zygarde: 186-222 (86.5 - 103.2%) -- guaranteed OHKO
252+ SpA Tapu Lele Moonblast vs. 252 HP / 4 SpD Zygarde-Complete: 186-222 (57.5 - 68.7%)

252+ SpA Tapu Lele Moonblast vs. 4 HP / 252 SpD Zygarde: 146-174 (79.3 - 94.5%) -- guaranteed OHKO
252+ SpA Tapu Lele Moonblast vs. 4 HP / 252 SpD Zygarde-Complete: 146-174 (50 - 59.5%)

The difference in damage is clearly shown, and this is due to HP:Def ratios skewing the general damage rolls in favor of Special Defense investment first, as Zygarde's Special Defense is lower than HP (especially on Zygarde-Complete). Of course, HP provides generic physical defense as well, but the principle still applies, and a spread can be optimized to account for a general reduction in both physical and special attacks using a tool like Survival Calc. Moreover, consider the % of HP remaining post-transformation with 4 HP and 252 HP. This is calculated by taking Zygarde-Complete's max HP – Zygarde-50%'s max HP + Zygarde-50%'s current HP at the time of transformation.
252 HP: 323-215+n = 108 + n
4 HP: 292-184+n = 108 + n
Where n is the amount of HP Zygarde had remaining before transforming.
Although both come out to be 108 + n actual HP, considering the percentage of HP remaining is more profitable for considering relative bulk. (108 + n) / 323 comes out to be roughly 0.33437 + n/323, and (108 + n) / 292 comes out to be roughly 0.36986 + n/292. n/292 will always be greater than n/323, and considering that 0.36986 > 0.33437, it is safe to say that Zygarde-Complete always has a higher percentage of HP post-transformation if it has a smaller maximum HP stat overall. Of course, it would be fallacious to assume that n is going to be the same in both cases. For example, suppose a Zygarde transforms at 40% HP; this makes n = 86 for 252 HP Zygarde, and n = 73 for 4 HP Zygarde (we will round down for sake of argument). Let's assume some arbitrary attack does 60% damage to both 4 HP and 252 HP Zygarde, leaving it remaining with that 40% HP (assumption for argument's sake).
252 HP: (108 + 86) / 323 = 60.062%
4 HP: (108 + 73) / 292 = 61.986%
Again, 4 HP proves to provide greater general bulk. When combined with superior HP:Def ratios, you can see that spreads with less HP are going to be generically bulkier on Zygarde.

EXAMPLE: Suppose you have a Zygarde with 100 free EVs that you want to put in generic special bulk. However, you also want regular Groudon's Precipice Blades to be a 3HKO. You've already met whatever goals you had in Attack and Speed. What's the best bulk spread available?
Here are the bulk spreads available for Zygarde-50% to accomplish the physical goal against Pblades:

252+ Atk Groudon Precipice Blades vs. 76 HP / 0 Def Zygarde: 81-96 (41.9 - 49.7%) (24 EVs remaining)
252+ Atk Groudon Precipice Blades vs. 44 HP / 4 Def Zygarde: 79-94 (41.7 - 49.7%) (52 EVs remaining)
252+ Atk Groudon Precipice Blades vs. 28 HP / 12 Def Zygarde: 78-93 (41.7 - 49.7%) (60 EVs remaining)
252+ Atk Groudon Precipice Blades vs. 0 HP / 28 Def Zygarde: 76-91 (41.5 - 49.7%) (72 EVs remaining)

Now, let's consider some arbitrary special attacks against our Zygarde. I'll pick LO Dazzling Gleam from Tapu Koko and unboosted Lunala's Moongeist Beam, but it could be any special move you wanted.
252 SpA Life Orb Tapu Koko Dazzling Gleam vs. 100 HP / 0 SpD Zygarde: 109-133 (55.6 - 67.8%)
252 SpA Life Orb Tapu Koko Dazzling Gleam vs. 52 HP / 44 SpD Zygarde: 109-127 (57.3 - 66.8%)
252+ SpA Lunala Moongeist Beam vs. 100 HP / 0 SpD Zygarde: 102-121 (52 - 61.7%)
252+ SpA Lunala Moongeist Beam vs. 84 HP / 12 SpD Zygarde: 100-118 (51.5 - 60.8%)
252+ SpA Lunala Moongeist Beam vs. 36 HP / 52 SpD Zygarde: 96-114 (51 - 60.6%)

Generally speaking, it seems we can increase bulk by investing in Sp. Def first, though of course the exact specifics are relative to the particular move we're looking at. Survival Calc says 52 HP / 4 Def / 44 Sp. Def is the best use of bulk EVs if our goal is being 3HKOed by Pblades, so I'll just go with that for the sake of the example. Let's compare the Zygarde-Complete damage rolls for 100 HP compared to this 52 HP / 4 Def / 44 Sp. Def spread.
252 SpA Life Orb Tapu Koko Dazzling Gleam vs. 100 HP / 0 SpD Zygarde: 109-133 (35.8 - 43.7%)
252+ SpA Lunala Moongeist Beam vs. 100 HP / 0 SpD Zygarde-Complete: 102-121 (33.5 - 39.8%)

252 SpA Life Orb Tapu Koko Dazzling Gleam vs. 52 HP / 44 SpD Zygarde-Complete: 109-127 (36.5 - 42.6%)
252+ SpA Lunala Moongeist Beam vs. 52 HP / 44 SpD Zygarde-Complete: 97-115 (32.5 - 38.5%)

As you can see, Zygarde-Complete also on average takes less damage. In addition, when it transforms, because it has less HP, it will have a greater % of health after transformation. This makes 52 HP / 4 Def / 44 Sp. Def a superior EV spread in this case.