Oh Armok. Did I not mention that you HAVE TO HAVE TO HAVE TO equip them with full armour? Mine do this only with a breastplate, mail shirt, greaves, gauntlets, high boots, and helm. I'm also working on getting the clothing industry running so they can have silk robes too. Thus equipped, the 1z drop is ineffective but the 2z works great with next to no injuries. I'm in the process of patching mine up to raise the level back up to 2 from the 3z I turned it into when I first started training swimming. Adequate swimmers after two years, so it's slow, but still it means they'll git gud one day. I have had two random bleedouts so far, and that's only after lowring it to 3z, so I can fairly confidently say that 2z is safe. BUT YOU MUST EQUIP THEM WITH METAL ARMOUR.
Anyway, dwarves are a very expendable resource. If your training isn't killing off a few in the process well then it's just not goddamned Spartan enough.
Just to check, when you say 2z seems to be the perfect drop, you mean that they're falling from the z-level they train on, through one level that is made up of only channels, and on to the bottom level where they land and return to their training? As I mentioned in an earlier post, I've been having problems getting them to take many hits when dropping them that distance. I'd reckon only one or two percent of the drops cause an actual hit, and the rest of them don't generate combat reports at all. And similar to your experience, increasing it to three levels of drop (which is to say, two levels that are channeled out, plus a top and a bottom layer) caused enough crushed ears that my prospective teachers had bled out/died to infections within a few seasons.