I tried again, changing the red decay density to 72%, and lowered the half-height from 2048m to 1024m. But as soon as I clicked on render preview, the half-height changed to 797.7m.
I presume Terragen isn't just picking a random value, and that 797.7 gives a different effect to 1024. But why?
Oh yeah, just got my Terragen keyfile through. You guys work fast!