It's not raws, but worldgen, I think. You can regulate the amount of purely stone-filled z-levels between cavern layers with advanced options.
This, and the higher a cavern's passage density/the lower openness, the 'higher' the cavern is, so it takes up more z-levels - which aren't bound to have much open space in them at all, so a high density, low openness cavern means even more stone.
I gen worlds with 15 levels before the first cavern, 5 levels between caverns, 3 cavern layers, high openness and low density and I meet the magma sea at z-level 70ish. Try it.
I'll give that a try and see what I end up with. Mastergilt, STRIKE THE EARTH!
PS: Wow, I died quickly. I didn't even start in an evil area... and I started off next to 4 skeletal alligators who chased me into the river filled with skeletal carp.
PPS: Okay, take two... a bit farther from the evil area. Swallowedgild, STRIKE THE EARTH!