UNDOCUMENTED MECHANICS
"Duration reduced by" above follows a diminishing curve, not a flat gain per level; the other numbers in the same block do not: 39% at level 1, 60% at 5, 70% at 10, 80% at 20, 90% at the limit. Reaching the limit takes skill level 60, gear bonuses included, so the first few points are worth far more than the last ones.