I listen to the Lord and Saviour Dervy Yakimi when it comes to stat weights, since the man does nothing but crunch numbers for this game when he has time to sit around. He seems to spend evenings just poring over data to determine optimal this that and the other thing. He spent the 3.1-3.2 downtime theorycrafting a 3-minute dummy parse for DRG. His weights are:
AP - 1
STR - 0.45
VIT - 0.45
DET - 0.139
CRT - 0.166
SKS - 0.117
Applying the latter three to Materia IV and V:
STR 5 = 6.750
STR 4 = 3.150
CRT 5 = 1.992
DET 5 = 1.668
CRT 4 = 1.494
SKS 5 = 1.404
SKS 4 = 1.053
DET 4 = 0.834
So, no, I organized them properly.
And, with respect to the Piety discussion... No. At first it seems like a good idea, sure, but if we assume no crafted anything, we're looking at 13 slots for Piety (because if you drop 15 STR for 15 PIE, I will simply not listen to your argument because it's wrong) which is a grand net gain of 195 Piety. Through my own testing, Piety scales at +4.3 MP per point on Dark Knight (the scaling differs by Job). So we're looking at a grand total net gain of approximately 838.5 MP - not even half of one Dark Arts.
And you are sacrificing 156 potential secondary stats in those spaces. Assuming that's all Skill Speed V (which is a pretty bad option), you're still looking at a minimum of +18.252 Attack Power that you're sacrificing for less than one half of one Dark Arts added to your Mana pool. And this says nothing of the fact that you're handcuffing yourself in which pieces to equip because you're melding zero Accuracy Materia.
I personally think that's absolutely ludicrous, but to each their own.
PS. +18.252 Attack Power is approximately equivalent to 3 additional Strength V Materia melds.
PPS. If it were all DET V, it would be 21.684 - if it were all CRT V, it would be 25.896. You would need 10/13 Materia slots to be Crit to make up 3 additional Strength V melds in stat weight.