I was wondering how close I can get to the Dark Suns setting? In other words metal poor with a world that is mostly, but not totally, desert?
From what I've seen..
If you set your rain value to 0-9, you'll only get deserts, badlands, or rocky wastelands.
If you have any rain in the world of 10 or higher, you'll get all those grasslands -> forest type biomes.
You can use the variance, mesh, and weighted range to affect what values within your rain range are used. A quick example would be if you set your Rain range 0-12, and then changed your mesh to 2x2 (in a 17x17 world) and your weighted ranges to 99,none,none,none,1 you would get very few 10-12 rain values used. However, if that were changed to 9,none,none,none,1 you would get more 10-12 rain values used.
As far as metal poor, you can set mineral scarcity to 100000 and you'll get less metals.
Examples:
One world tile of grassland (10+ Rain):
worldgen
Created in DF v0.47.05.
[WORLD_GEN]
[TITLE:RAIN1]
[SEED:kcogEGmcSu8o2Q2giIes]
[HISTORY_SEED:quK4uQKuMeOyiCMYs62Q]
[NAME_SEED:YukCaOmsAUECi4gUMo6a]
[CREATURE_SEED:uMqw20IC2Icm6eQUsCsG]
[DIM:17:17]
[EMBARK_POINTS:0]
[END_YEAR:200]
[BEAST_END_YEAR:200:-1]
[REVEAL_ALL_HISTORY:0]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:100:300:400:400]
[RAINFALL:0:12:20:20]
[TEMPERATURE:75:75:0:0]
[DRAINAGE:0:0:0:0]
[VOLCANISM:0:0:0:0]
[SAVAGERY:0:0:0:0]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:2:99:0:0:0:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[POLE:NONE]
[MINERAL_SCARCITY:100000]
[MEGABEAST_CAP:0]
[SEMIMEGABEAST_CAP:0]
[TITAN_NUMBER:0]
[TITAN_ATTACK_TRIGGER:0:0:0]
[DEMON_NUMBER:0]
[NIGHT_TROLL_NUMBER:0]
[BOGEYMAN_NUMBER:0]
[NIGHTMARE_NUMBER:0]
[VAMPIRE_NUMBER:0]
[WEREBEAST_NUMBER:0]
[WEREBEAST_ATTACK_TRIGGER:0:0:0]
[SECRET_NUMBER:0]
[REGIONAL_INTERACTION_NUMBER:0]
[DISTURBANCE_INTERACTION_NUMBER:0]
[EVIL_CLOUD_NUMBER:0]
[EVIL_RAIN_NUMBER:0]
[GENERATE_DIVINE_MATERIALS:0]
[ALLOW_DIVINATION:0]
[ALLOW_DEMONIC_EXPERIMENTS:0]
[ALLOW_NECROMANCER_EXPERIMENTS:0]
[ALLOW_NECROMANCER_LIEUTENANTS:0]
[ALLOW_NECROMANCER_GHOULS:0]
[ALLOW_NECROMANCER_SUMMONS:0]
[GOOD_SQ_COUNTS:0:0:0]
[EVIL_SQ_COUNTS:0:0:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:1000]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:0]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:1]
[CAVERN_LAYER_OPENNESS_MIN:100]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:0]
[CAVERN_LAYER_WATER_MIN:25]
[CAVERN_LAYER_WATER_MAX:25]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:8]
[LEVELS_ABOVE_LAYER_1:8]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:1]
[CAVE_MAX_SIZE:1]
[MOUNTAIN_CAVE_MIN:0]
[NON_MOUNTAIN_CAVE_MIN:0]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:1]
[TOTAL_CIV_POPULATION:0]
[SITE_CAP:0]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
Several world tiles of grassland (10+ Rain):
worldgen
Created in DF v0.47.05.
[WORLD_GEN]
[TITLE:RAIN2]
[SEED:yEcppNbxtZ4ykxisMjRP]
[HISTORY_SEED:lpzwvry5OzdiQ4vD6UjS]
[NAME_SEED:QinGnmkKLi7JmVT0RMgb]
[CREATURE_SEED:kk7kK23MEia1KJ2tdWAe]
[DIM:17:17]
[EMBARK_POINTS:0]
[END_YEAR:200]
[BEAST_END_YEAR:200:-1]
[REVEAL_ALL_HISTORY:0]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:100:300:400:400]
[RAINFALL:0:12:20:20]
[TEMPERATURE:75:75:0:0]
[DRAINAGE:0:0:0:0]
[VOLCANISM:0:0:0:0]
[SAVAGERY:0:0:0:0]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:2:9:0:0:0:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[POLE:NONE]
[MINERAL_SCARCITY:100000]
[MEGABEAST_CAP:0]
[SEMIMEGABEAST_CAP:0]
[TITAN_NUMBER:0]
[TITAN_ATTACK_TRIGGER:0:0:0]
[DEMON_NUMBER:0]
[NIGHT_TROLL_NUMBER:0]
[BOGEYMAN_NUMBER:0]
[NIGHTMARE_NUMBER:0]
[VAMPIRE_NUMBER:0]
[WEREBEAST_NUMBER:0]
[WEREBEAST_ATTACK_TRIGGER:0:0:0]
[SECRET_NUMBER:0]
[REGIONAL_INTERACTION_NUMBER:0]
[DISTURBANCE_INTERACTION_NUMBER:0]
[EVIL_CLOUD_NUMBER:0]
[EVIL_RAIN_NUMBER:0]
[GENERATE_DIVINE_MATERIALS:0]
[ALLOW_DIVINATION:0]
[ALLOW_DEMONIC_EXPERIMENTS:0]
[ALLOW_NECROMANCER_EXPERIMENTS:0]
[ALLOW_NECROMANCER_LIEUTENANTS:0]
[ALLOW_NECROMANCER_GHOULS:0]
[ALLOW_NECROMANCER_SUMMONS:0]
[GOOD_SQ_COUNTS:0:0:0]
[EVIL_SQ_COUNTS:0:0:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:1000]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:0]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:1]
[CAVERN_LAYER_OPENNESS_MIN:100]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:0]
[CAVERN_LAYER_WATER_MIN:25]
[CAVERN_LAYER_WATER_MAX:25]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:8]
[LEVELS_ABOVE_LAYER_1:8]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:1]
[CAVE_MAX_SIZE:1]
[MOUNTAIN_CAVE_MIN:0]
[NON_MOUNTAIN_CAVE_MIN:0]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:1]
[TOTAL_CIV_POPULATION:0]
[SITE_CAP:0]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
While the rest is desert, setting aside lakes.
Aside from the seeds, the only thing different in those two worldgens above is the Rain Weighted Range (0-20) changing from 99 to 9.
You can remove the seeds yourself and generate many worlds with those two parameter sets, and
generally, the above appears to be true.
Generally.