Rant: I stopped guessing where to drill and started checking the stud finder twice
I used to just eyeball it and start drilling, which cost me plenty of drywall patches and one nasty nick in a coax run behind a bedroom wall in Beaverton last year. About 8 months back I picked up a cheap stud finder with a deep scan mode and started running it in two directions before I even mark the spot. The difference is night and day, I probably saved 2 hours a week in patching and touch up paint alone. One job in a 1970s ranch house had crazy thick plaster and my old method would have put a hole right through a water line. I still see newer techs on my crew skipping this step because they think it slows them down. Spend the 30 bucks and 20 extra seconds, your drywall guy will thank you. What is your go to trick for finding a clean path in old walls?
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.