I just upgraded my TiVo and now I want to be able to figure out how many hours of recording time I will have at the higher quality levels. I now have "variable up to 421 hours" of recording time. So I want to know how much time I'll have at Best and High quality. Is there a formula or rule of thumb for the different quality levels? Or a way to relate hard drive size(es) to the amount of recording time at each quality level?