I have not leveled my tanking jobs to 50 yet and most likely won't use them in group play even when I do. Just not my thing. However, I'm very interested in stat theory, and I got to thinking about reusing i70 crafted/melded DPS accessories for a tank, because they come with STR and DEX, and you can just meld in VIT. Whereas tank i80/90 pieces are VIT only. And i70 crafted tank accessories aren't as effective as DPS ones when you try to max out STR and DEX via melding.
So, the difference in VIT between an i70 HQ and i90 accessory is 5. That's a total of 25 VIT (375 hp) across all 5 slots. However, for each i70 piece, you'd get 9 DEX and 9 STR, a total of 45 of each with all 5 slots. STR and DEX help with block/parry effectiveness and activation rate, but only slightly. I've read up on testers identifying stat tiers where these are pushed up by 1%. It is safe to assume then that you'd hit 1 and possibly 2 1% tiers of amount blocked/parried and activation rates from the STR and DEX, as well as deal more damage/threat/self heals.
Whereas the stat Parry seems to be around .077% Parry rate per point, with no tiers, according to a reddit source. So 13 Parry = ~1% activation rate, if the source is to be believed.
I'm trying to guage the opinion Tanks about this i90 set (picked bc of maximum Parry and decent acc):
Allagan Earrings, Allagan Ring, Hero's Ring, Allagan Choker, and Hero's Bracelet
VIT: 75
Parry: 80
Acc: 38
Crit: 11
SkillSpd: 11
Vs
This set:
Gryphonskin Ringx2, Choker, & Bracelet, Rose Gold Ear Screws. Each melded to have 10 VIT (1x IV, 1x II), 12 Parry (2x III), and 1 other tier III materia.
STR: 45
DEX: 45
VIT: 50
Acc: 57
Parry: 60
Det: 28
Crit: 18
SkillSpd: 18
The VIT/Parry melding can be achieved with just 4 materia. Each piece could hold 1 more materia providing a little more crti/det/skillspd/acc. If it were me, I'd probably make them be 2 det III's and 3 crit III's. Acc is capped or nearly capped already on these items and I think SkillSpd would just make you burn through TP a little faster. But I'm no tank, just a guy working a theory.
You can build these i70 accessories pretty easily (albeit pricey) compared to farming the i90 drops and myth tomes, get some great accuracy in the process, and amp Parry/DEX enough that they should be able to match the Parry/Block rate of the best i90 Parry accessories. Meanwhile you'll block/parry a tiny bit more damage, dish out a little more hate, and deal a little more damage because of +45 STR. Which incidentally, also ends the fights a little faster and helps your party's DPS make the grade whenever they're checked. When using WAR abilities that heal based on damage dealt, you'd also recover a little more hp because of the STR.
So the actual comparison is 25 VIT, 20 Parry VS 45 STR, 45 DEX, 19 Acc, 28 Det, 7 crit, 7 SkillSpd. If you keep in mind that the Dex should have about the same effect as the Parry, then all your really comparing from the i90's is 25 VIT.
There are a lot of ways you can mitigate damage/lessen required healing- whether by self healing, killing faster, natural hp ticks stemming from VIT, or blocking/parrying. And the bottom line of course is the time you survive coming below 375 hp is the time 25 VIT wins hands down.
What do you think?