70% is a good place to start. I get higher efficiencies since I started batch sparging, more like 80-85%. I don't know where the rumor about poor efficiencies comes from, maybe people don't sparge enough or stir the mash enough.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk