Different surgeons use different cut offs but the number i see most frequently is 35%.
That is, your gb must eject at least 35% of its volume of bile
to be deemed normal. Less than 35% may mean there is an obstructive process (stone? Sludge? Spasm? Inflamation?) that is preventing this ..And thus causing pain. It is usually employed when the us did not reveal a cause of pain like a stone.