Interesting question! I would be surprised of many people systematically check their tailings regularly enough to be able to answer. Which is unfortunate really as testing your tailings is helpful imao. I’ve thankfully never seen much in any tailings testing I have done. But do test reasonably regularly when I remember to.
You’d have to think you’d have something wrong to only catch 40% in the sluice as the article says but maybe I’m wrong. I can imagine using a dredge losing a decent amount if its set up weirdly. I’ve seen a set up with someone not using a damper, where they likely lose a bunch of gold when it was running the pump pretty high, for obvious reasons. That damper is kinda important piece on a dredge to force the water down on the riffles! But does it count if you have something not right? I’d like to think even that would catch more than 40%?
I think would be happy with anything higher than 90%. But def not happy if I lost any big bits. Normally you have a decent feel dredging what you are seeing. So I dont think I am losing more than that. Great question!