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Hi Mike,
 
I believe cumulative latency is added up... then everything else is delayed by that amount.
Thus, it doesn't matter where in a project a latent effect is added.  If it's in the project, all tracks are affected.

I guess I'm imagining that if they are running paralell the worst case may just be equal to the longest solo instance. I'm wondering what happens if two effects that appear to be running parallel are actually running more as if they are patched sequentially? It seems like a fairly difficult juggling act to keep the final output in sync once someone starts placing latency inducing effects on more than one track or bus.

 
You got it on all accounts... 
I'm glad I'm not the one who has to come up with the math for automatic PDC.