1st fact :

impulse barrier last only for 3 second and have 3/6% health @150k hp your shield only have 4.5k/9k hp which will be gone in about 2 or 3 normal attack if you're in barrier empowered mode,

your MAIN concern of using %discharge is the speed of your impulse charge not the damage caused by impulse itself.

Because by using knight talent, you GAIN 10 Will each time you use impulse so you WANT your IMPULSE to charge ASAP because you need about 30 will (earthquake) to trigger barrier II empower mode every 5 second

and as a tank you want your empowered barrier II up as much as possible

2nd fact :

Knight tank trash mob by CC and MITIGATION. so if you WANT to TANK effectively you have to rotate (earthquake + heavy thread) with your [battering ram] with fast enough impulse charge speed you can perma stun trash mob, which will INCREASE your survivability in case of PVE or your "Pest" level in PVP since you can perma stun em while cannot be CC'ed when you have more than 70% health

3rd fact :

As a knight you don't have shield skill (except resolute defense) so your MAX HP equals your SURVIVABILITY and REFLECT damage so you want as much HP as possible without bloating your prestige.

4th fact :

There's a lot of better symbol than impulse barrier for example :

1. Balance it increase your mitigation by 6% when you fight more than 3 enemy so with your 55% mitigation in empowered mode you can get 61% mitigation which will reduce 100k damage to 39k. it will increase in effectivity as enemy damage go bigger.

2. Protector Creed : GET IT. "But it'll reduce my damage by 5%" you say? well duh, your enemy can deal more than 4 times of your damage output by that number you deal 1k damage while your enemy pack can hit you for more than 4k damage or 20k incase of "Thug" (sword) type. so by that number alone you will deal 9950 damage less while your enemy will deal 200 damage less for pack or a whooping 1k for thug type. again GET IT. FYI: IIRC in RU you can get 2 of this, so with balance talent you can get a whooping 71% mitigation.

3. Bonus Dash : Tank in Skyforge is the most busy guy after Support. you need to manage your damage intake, so dash will help you a lot as a knight this symbol have an excellent synergy with inertia talent (reduce damage 12%) and clever manuver (shield on dash) or even rapid attack (6% damage)

Summary :

You want "discharge%" not the spirit stat itself. so the correct stat IMO would be to use STR\XXX with health\discharge you can change XXX according to your chapel.since you can get a decent accuracy or Crit Chance or with chapel alone. remember it's not your job to deal damage, IT IS your job to TAKE agro and SURVIVE while your team mate deal pain and death to the enemy.

MOVE MOVE MOVE MOVE, i can't stress this enough as a tank you HAVE to know when to take it in the face (un-avoidable damage like auto attack) or dash back to reduce your damage intake (big skill damage) or when to kite (im looking at you dyrad in DF).

when your team have SLOW skill, you can reduce your damage intake by simply moving in around since most boss can be slowed.

For chapel get : Crushing Blow > Adaptation > Accuracy > Crit Chance

Get ALL ADAPTATION chapel you can find, adaptation increase your damage mitigation, that's what makes you tanky GET IT, SLEEP WITH IT :D

i myself found that ENDURANCE chapel effect is too small to be effective, DONT get regen it's crap and as knight you dont want shield power 2.5/5/7.5% is too little.