So, first off, you should be able to discriminate out the bottle caps by SOUND alone. There's no reason for you to be digging them, or attempting to notch them out.
If you're using the 5x8, wiggle it backward from the target until you just barely catch the edge of the bottle cap. It'll grunt iron...Every time. Bottle caps have a decent amount of steel in them. There is a lot of it, which is why they ring up high, but if you catch the lip, you get a true accounting of the metal content, hence the grunt (and if you have iron discrimination turned off, as I do, you can just watch the VDI for it to drop below 40 (sometimes as low as in the teens).
Put down a bottle cap and a quarter about 18" apart. Swing over them both until you can notice the difference. It won't take long, and it'll save your back a TON of up/downs in the park. Unless you're literally trying to clean up the trash, there's no point in digging the cap.
Also, you should make sure your sound is up high enough (not blasting your ears), but high enough you can catch the differences in sounds. this will require you to have your machine volume on at least 4, and then adjust your headset volume accordingly. I've run machine volume as high as 7, but fine tune the headsets.
What you're looking for is the subtle "woop" or "wooow" of aluminum slow, over a "poop" or "boop" of a ring/jewelry.
It's hard to describe, and you absolutely can discriminate out more trash than you think. Also, rather than notch out any particular group of numbers, it's far better to "sound out" the items. This will help you learn your machine better, too. For example, when you get to a park, you should always dig the first 20 or so targets. This will help you get a feel for the park conditions on that day. It'll also help you catch the sounds the various common trash makes.
What you're looking for after that, is anything DIFFERENT from those sounds. And by not notching them out, you continually come across the sounds, and you train your ear to hear them over and over. Anything different should be dug.
For example, I know that folks say "you can't tell the difference between a pull tab and a ring." And that's true, but not all 52 signals that are good sound like Pull Tabs. Which means you're missing actual good signals you can tell the difference on, simply because you've notched it out.
Basically, leave all your notches on from 38 and up. I find LOTS of gold rings in the 40's. Don't discriminate out this range, if you want to find them. Dig any signal you're confident is either different from the signals you're used to hearing, or is "different" enough that you want to investigate.
Eventually, you'll be discriminating out LOTS of trash, without having to actually notch it out. This gives you the best shot at catching good stuff, even if you're skipping pop tabs.
That being said, trashy parks will still have lots of trash end up in your pouch, but you won't be missing the good stuff.
Hope this helps!
Cheers,
Skippy