If you know about how big a microcline (or whatever) patch is that is basically the biggest pillar of rock you can find to dodge a cavern layer through. If you were to flatten a cavern it's got a general trend of having about a third of the space full of rock chunks in that general size range, a third being big open patches, and a third being these squiggly little tunnels. Those last two are definitely not evenly spaced though so any particular site will have more of one than the other. If you are just making a single tile stairway you'll pretty often sink it through a little spot between the squiggly passages.
If you're really set on planning it out after the first few layers just send a miner off to some corner of the map and have him dig down and around till you find the cavern then tell him to dig little spots in the walls of the cavern so he'll run out and explore. There are only a few creatures do there you really need to not run into to make it back out alive, and you have a good shot of none of them being present right at the start. Then once you have mapped out the cavern just let him come back up and stick a floor over the stairs he dug. It spoils the surprise of digging into the place but it sounds like you don't want surprises so much. Or if you do there are always layers two and three.
Note: if you generate islands you tend to get like 20 z levels down to the caverns and tons between them. It's hard to find embarks with less than 50 levels to the bottom on those. I guess the caverns start at the edges of the map and with deep oceans there it is hard for them to migrate up to where they are supposed to be. No-edge-ocean embarks have much more predictable cavern depths so you can just set it to about what you want in world gen.
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Topic: [minor spoilers] Do I have guaranteed room to plan? (Read 1960 times)