For those kinds of things, I'd recommend measuring the length of the worst-case stuff you have run across or expect to, and make sure there is more rubber between your air and the road than that.
Protection strips like the slime or other brands can help deflect the pointy things to the side of the tube, but they need to be able to deform to do this--so they have to be close to the tube itself, like just one innertube rubber layer away from the tube itself at most. For instance, if you used the tire-in-tire method, and put the strip between the tires, they may clamp the strip so tightly that it cannot "slide out of the way" of the tip of the pointy thing, to start the deflection process, and in that event the pointy thing can stick into and thru the strip instead.
I discovered this originally while trying to prevent the "snake squiggle" problem--where the strip over time migrates into a sinewave-like pattern crisscrossing the tread area of the tire, migrating up the sidewalls and no longer protecting against tread punctures over the whole tire. I glued the strip to the tire's tread area, on the inside of the tire. Then installed the tube normally. Strip would still stop "bendable" things like wires shucked off of steelbelted tires and whatnot, but only because they would just bend and squish as I rode over them. Roofing nails, shards of glass, etc, would jam into the plastic and be pushed all the way thru it and into the tube as the tire rolls over them the rest of the way...and the longer nails, being held verticaly by the strip and tire, then *also* go thru the rim!
The tire-in-tire does a similar enough thing to the strips as the glue to have the same effect, when tire pressure is normal (really low pressure might let the strips work but I don't know how well, and the tire could get pretty hard to roll with all the rubber layers in it at low pressures).
If the strip is right up against the main tube that holds the air, it works best, as it can then bend a bit away from the incoming pointy thing and cause the pointy bit to slide left or right away from the tube (and out the sidewall instead, if it's long enough).
But...these strips are known for having edges that can cut or wear thru the tube and cause a flat themselves--not just a simple patchable puncture either, but a long cut or rip.
So...use a dead tube cut along it's inner circumference to cover your real tube, then put the strip just outside that, and then all the other rubber layers then the tire, and it should still be able to do it's job.
