It is linear from 100% to 0% as far as I can tell. Typically if it starts scaling at a certain point you'll see the tooltip mention that like with executes, the Bloodthirsty trait, and the old Undeath that started scaling at 50% HP. Eviscerate/Arterial Burst have the same scaling as Dragonknight's Coagulating Blood which begin growing in power the moment you have missing health.

edit: This means at 50% HP Arterial Burst will do 16.5% more damage after the 3.3% bonus damage for having the ability at rank 4, and Blood for Blood at 50% HP will do 37.5% more damage but doesn't have Arterial Burst's base 3.3% bonus damage as leveling it up boosts the damage scaling based on missing health instead.

Arterial Burst at 50% might deal a little less damage at that health range than Blood for Blood, but below that it'll have a 100% critical chance over Blood for Blood which will have whatever your spell critical chance is upon casting it. Means in PvE Arterial Burst has an advantage over BfB while not having to lock you out of outside healing, but in PvP where critical hits are not as good Blood for Blood is quite devastating to your target. Biggest issue with BfB is allies as they can't heal you, and if they're grouped with you the Pale Order Ring you're probably using to fuel the ability is made worse per person you're group with till you have 5 people in your group and now the ring is worthless.