My back-of-the-envelope numbers are close to yours, but I think a very big caveat is the confidence of these associations from epidemiological studies. 18% percent is quite low by most standards. Most epidemiological studies expect 100%-400% to draw strong correlations. Smoking, by example, is in the thousands of percent increase risk.
"In adequately designed studies we can be reasonably confident about BIG relative risks, sometimes; we can be only guardedly confident about relative risk estimates of the order of 2.0, occasionally; we can hardly ever be confident about estimates of less than 2.0, and when estimates are much below 2.0, we
are quite simply out of business." [1]
The estimate of 1.18 would probably be regarded as low. While the numbers we calculated may be nothing to sneeze at, I think we should be extremely cautious about our confidence in interpreting those values to real-world conclusions.
The 18% and 4/5 in 500 is just an example I pulled out of my butt. I didn’t do a scihub search to find the study but 18% is pretty weak. It’s also more for an assessment at a personal level how much risk you could expect as an individual, which most people care about. Even if that means 600k more cancer patients than before, you’d have to compare that against the null hypothesis to even see if you’re still in the territory of what random chance could arrive at.
An example I can off the top of my head remember is related to how much risk there is for women to have children post 40. It is a 100% increase in birth defects. From 0.5% to 1.0%, sounds bad right? Well its out of like 100 000 people and was based off of 1600’s era French women. Always take studies like this with a grain of salt and look at the numbers to assess personal risk.
Making lifestyle changes purely off of these studies is premature in my opinion. But you do you.
That's nothing to sneeze at.