Plains percentage not working?

I’m trying to make a sort of windy canyon biome. I’ve set the plains percentage to things like 99.5%, but I’m not getting any plains, just foothills. I can’t seem to figure this out - does anyone have an idea as to how I could go about making this?

Here’s my mod for reference: ems_biome_windy.smod (1.0 MB)

foothills are counted in that plains percentage, so you have 0.5 (stone) mountains.