Wait, are you really saying a cheap stud finder handles thick 1970s plaster? Because that has not been my experience at all. Those cheap units get confused by the wire lath and the plaster buildup, and half the time they light up on nothing or miss the stud by an inch. I've had way better luck with the old school method on that kind of wall, drilling a tiny pilot hole at an angle and feeling for the stud, or using a strong magnet to find the nails holding the lath. The magnet trick works great because it locks onto actual fasteners instead of guessing at density. And that deep scan mode you mentioned is exactly what tends to give false readings in old walls, so I'd double check those marks with a magnet before you drill.