Can you elaborate that? For each BGA ball, I need one trace width of trace, plus one spacing width to the next trace, right? There's a space on the other side as well, but that's already 'provided' by the trace above.

Or do you mean if you want to connect to both the outer as well as the inner layer of balls? Then you'd be right I think, but that was not what I meant; I only calculated the spacing to connect to the outer layer and nothing more.