Hi
You have many options how to scatter on large areas and fix tiling that comes from default distribution modes.
1. Add Random translation per item using Transform Rollout:
![](https://i.imgur.com/PKjmo4s.gif)
2. Use custom procedural map, as You asked.
![](https://i.imgur.com/qkeRaBN.png)
3. Use particle flow as distribution:
![](https://i.imgur.com/4xw2CaF.png)
And probably some other methods to fit Your needs.
Anyway I would recommend You
watching this tutorial which explains ForestPack Distribution settings and shares many useful tips.