I asked Claude to research for me so don’t take this as truth but it sounds like

(a) as the levels drop below 300M barrels, pumping out of the reserve is much less efficient, so we can’t necessarily draw it down as quickly as when it’s full. So the lower it gets, the less useful it is

And (b) there are a lot of varying estimates for the real effective floor, anywhere from the 180ish to 300M barrels is within the range of a safe minimum level.