The most you would need to do, assuming you're not using the passive stat relics, is 4-5 (full 27 unlettered mods, full Bastion/Bulwark enhancements, and high mitigation ear and implants provides you with 600 Shield exactly, with a stat budget of 1648 without using passive relics). I've also got a bit of a problem with using those exact numbers because their interplay with the various relics and debuffs: the presence of the acc debuffs, especially the 20% from Smoke Grenade, is gonna do some wonky things to those numbers when factored in (especially when factored in when weighed based on uptime rather than averaged for effect). It's one of the reasons why, even though I know about these calcs and think they're a respectable guideline, I don't follow them explicitly (my Shadow optimal loadout is 532/600/516 rather than the 500/525/600 that I "should" be aiming for).
I'm using passive pvp relics in all fights with except ones where two tanks transfer a single boss (both versions of Kephess and TFB actual). I'm working on a calculator that calculates mean mitigation factoring in uptimes for cool downs and relics, the formula for relics is very much in favor of the PVP relics in fights with high tank uptimes. I'm using one shield and one defense pvp relic usually.
The math for relics is a bit complicated and an eyesore, it can be found in one my my spreadsheets posted earlier however.
The built in tech tank accuracy debuff is a very good point that I'm going to adjust my spreadsheets and the program I'm writing to account for. Should be mathematically equivalent to adding the debuff to the total defense percentage.
Edit: the relic formula I am using is weighted for uptime, I agree 455 absorb on a 6 of 20 proc relic does not equate to 455 * 3 / 10 added to your absorb. Rather it is (MMactive * duration / cycletime) + (MMpassive * (1- duration / cycletime)). This can be extrapolated out to include multiple relics and cooldowns. You can see where the formula gets huge with multiple cd's and the fact that MM numbers are a really long formula themselves.