setting coefficients for baseline case

I think your question illustrates exactly the reason the USGBC hasn't established clear guidance as to what is conventional or typical in various regions or settings. Practices may differ widely based on applications, context, etc... and there simply aren't hard and fast rules out there that anticipate all these variables. I think your best bet is to do some legwork to establish what common practice in your neighborhood, in similar applications/buildings/settings is and document your conclusions for the reviewer. That being said, though I can't speak to roof gardens, I would think that turf grass is considered a typical/conventional landscaping practice pretty much anywhere in the US. Hope that helps a little. . .