The IARC grouping isn't for effect size though, it's for certainty about carcinogenicity outcomes with exposure. It's not going to tell you how much exposure you need to get cancer. There are different measurements for that.

IARC Grouping Levels: Group 1 - We are certain this will cause cancer Group 2 - Probable it causes cancer Group 3 - We don't know if it causes cancer Group 4 - Unlikely to cause cancer

So when looking at something like tobacco smoking, we have lots of evidence that people who smoke get lung cancer. So it doesn't fit in group 2,3,4. So it goes in group one.

For scales that measure exposure needed for negative outcomes, most of the time these are for chemicals that are/have been used in work environments. So EPA (and probably OSHA) has a threshold scale for benzene, but not for tobacco cigarettes. Most of the time, there aren't MSDS (Material Safety Data Sheets) for consumer products like cigarettes.

But really, what is more likely - a person getting cancer from tobacco smoking or benzene? I would offer that tobacco is more of a danger than benzene d/t the easily available nature of it. So in practice, tobacco is more of a danger than benzene.

The LD50 of benzene is 980 mg/kg; drinking that much benzene every day would undoubtedly be fatal