Lv45: -20% HP lost (8%) ~ Someone could confirm this /DRK easy.
Lv60: -30% HP lost (7%/8.4%) ~ Extrapolated. Second number is with AF helm
Lv75: -40% HP lost (6%/7.2%) ~ Confirmed with Turin's numbers.
Lv90: -50% HP lost (5%/6%) ~ Confirmed with my own numbers
So subbed at 90 would give 5% to damage for 8% lost. Exact levels could be stupid, but I would expect this distrubution. This gives us Stalwart Soul IV at 90.