@Namasu

You need way too much Dex for Darksteel Scutum to overcome the Block Rate iLvl 90 ones provide. based on my observation, not definitive test, that 1 Block Rate = 1 Dex. The scutum would increase your block strength by 3-4% however it will make your Shield Block way less capable of reaching 7/8 block it will probably go down to 6/8. And as far as Coil 5 goes blocking that extra 3-4% doesn't matter... Block it matters. As an advice you would want to based your test hovering around 247 Dex decreasing and increasing around that value.

As a gladiator/paladin you would want to get more Dex till 247 after that you really have no option of getting any more as far as iLvl 90 restrictions go. Next would be Parry rating.