Great that you’ve got it working! Basically the ‘Set this to the Hight’ marked value must be greater than the first one, and if you place ForestMaker right on the ground by drag & drop, then it should work even with values like 10000 (‘Set this to the lowest’ value) and 15000 (‘Set this to the Hight’ value) - but yeah, the more diverse (in terms of height values) your terrain is, the more you should put in this second value. I’ll fix it in the upcoming update for 4.13, to make it more convenient.
Thanks and good luck with your project!