there are 18,000+ manga in the database, and finding accurate chapter counts is only possible for a small percentage. There's no way we can come up with the kind of custom solution you're looking for with this scale, hence why it has to be a rough estimate. What you suggest still isn't a one-size-fits-all solution and would still be inaccurate, as well. Often vols that have 12 chapters are because the chapters are really really short (10 pages or less), often with comedy series. So if we somehow were able to get chapter info for all 18,000+ series (literally impossible with the vast majority), giving time based only on chapter count would be equally as inaccurate. The only way to do a 100% accurate solution would be to manually set some sort of time/duration for all 18,000+ manga individually. Coding that would be a complete nightmare with extremely low payoff in comparison, but beyond that we don't have the massive resources needed to actually go fill in all of that data, test it, etc. tl;dr I realize it's not 100% accurate, but hopefully this strengthens the reasoning of why it's just not possible to do it any other way.