Sorry I can't be of much more help, as I've never used UVDeluxe really, but I think you might be over-thinking the tutorial. The value 204 seems pretty arbitrary. If you weren't following a tutorial you would determine your own pixel density based on the requirements of your project, similarly to how you would decide on…
This is an oil lantern I made for an assessment. 1024 x 1024 textures. I think I would have been better texturing at 2048 and then dropping the resolution. Comments and criticism welcome.
You may think about how many different peaces of rubble you really need and only use these as particles instances. Maybe even merging and makeing a convex hull and bake the texture to that "one-filled-piece-of-rubble". Or simply do a shrinkwrap of a grid on top of the particle simulation. Ohh.. and someone did this already…