... Edit again, for knowledge's sake:
How did you manage to generate a world with mountain peaks (necessary for dwarven society) when max altitude was 350, and without setting the minimum number of mountain peaks?
From what I've seen, the X and Y variance values are essentially distribution values. My impression is, the lower the value, the more adjacent the distribution (1 1 1 1 1 1 1) whereas the higher the value, the more varied the distribution (1 100 1 100 1 100). The variance will use the min and max of each (in this case, elevation) to produce it's values.
I use 350 just to get mountains, but not peaks. Despite what some people claim, a peak is not
required for the mountainhome, just a mountain biome. Given you can't grow crops on a mountain biome, I'm not a fan of them for embark locations, so I try to keep them to the absolute minimum required. You could use 301 max elevation, and it would potentially work, but you might get a few more rejections until you got a few adjacent mountain tiles.
The other method is to use a 10% max elevation weighted range for the high elevations. That is, 9,0,0,0,1, so 90% of the elevation is within 0-20% of the distributed values, none for 20-40, none for 40-60, none for 60-80, and then 10% for 80-100%. What this means in practice is 90% of the world will be lowland, and only 10% will be highland. If you use no variance with this, you can make a very flat thin world for most of it, and give the mountainhome it's 10% mountains it needs to start.
Of course, with a single line in the raws, all of that is unnecessary and dwarves will use any biome, but still... gotta keep the vanilla folks happy.