I've been playing around with the layer options and I thought others might find some of the info useful:
[CAVERN_LAYER_COUNT:3] <- Number of cavern layers present. Each layer is a multi-level cavern system.
[CAVERN_LAYER_OPENNESS_MIN:0] <- Seems to make the open areas bigger with higher (multi z-level) roofs.
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0] <- Seems to affect the number of passages in the maze-like areas. If set to 100 these areas become just scattered pillars.
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0] <- Determines min amount of water present in caverns. If set to 100 caverns almost entirely water.
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1] <- Yes/No, determines whether or not magma layer is present. Forced to Yes if bottom layer is present. If no, levels end abruptly at end of last cavern layer (lot less Z levels).
[HAVE_BOTTOM_LAYER_2:1] <- Yes/No, determines whether or not bottom layer is present. If No, replaced by layers of standard rock (doesn't reduce Z levels). Also determines if adamantine is present in layers above.
[LEVELS_ABOVE_GROUND:15] <- Empty levels above ground (useful for tower construction)
[LEVELS_ABOVE_LAYER_1:5] <- Stone layer before caverns (fort building/mining space before you reach caverns)
[LEVELS_ABOVE_LAYER_2:1] <- Stone layers between cavern layers 1 and 2 (does not affect size of cavern layer itself, can contain connecting tunnels between cavern layers)
[LEVELS_ABOVE_LAYER_3:1] <- Stone layers between cavern layers 2 and 3 (same as above)
[LEVELS_ABOVE_LAYER_4:1] <- Layers between cave layer 3 and magma layer (can contain magma shafts)
[LEVELS_ABOVE_LAYER_5:2] <- molten rock layers between magma lake and bottom layers (contains adamantine shafts, otherwise useless)
[LEVELS_AT_BOTTOM:1] <- number of "fun" layers, containing slade and friendly dwarf-loving critters
Going no magma and no bottom layer cuts the z-levels down quite a bit, so someone on a low end computer could up their performance that way if they didn't mind missing out on magma and some HFS (or if they settled on a volcano). Reducing the levels above ground helps too (less sky), but would prevent you from building upwards (tower or whatever). Reducing cavern layers can cut down on z-levels too, but you miss out on the potential fun of cavern systems. You could get by with 1 cavern layer and still get migrating underground fun.
An example 1 cavern system map came out with 14 z-levels. 1 sky, ground, and 12 below ground. 6 of them were the cavern system, one was a deep rock layer below the caves, and the other 5 were soil and rock above the cavern system.
Reducing everything to minimum and cavern layers to 0 is not advised unless you want a non-digging farming simulation or something, since it gives you a grand total of 3 z levels - sky (which walls can't be built on), ground, and soil underground. On the plus side, even an old crappy computer could probably run a decent sized fort with so few levels haha. It might be interesting playing on this in a forest by a river and just importing everything, building yourself a castle out of imported stone or local wood (could increase the sky z-levels to allow building more than 1 level up).
Things are obviously going to vary based on the site picked though. If you pick a flat area you'll get less z-levels than a cliff, and picking a site beside an ocean seems to give you a really deep underground (the 1 cavern map has places where that one cavern goes 100 z-levels deep, where otherwise its only 11).