Without my helm equiped, i have 423 SBV from gear + 176 from str= 599
599x1.3 from shield mastery gives me 778.7 which my character sheet shows 779.
now enter the helm:
it has 26 str, and when i mouse over my str on my character sheet it gives me 189. (now right here thats red flags because vitality adds .78 shield block value, which is now lost, but back on track). With the helm on, my character sheet now says 826.