How much with-the-rule astigmatism would you intentionally leave in patients under 40 undergoing cataract surgery, considering the expected age-related shift toward against-the-rule astigmatism?

IOL selection and expectations are hugely important in this conversation. Will leave less for a multifocal, and more with a standard IOL. A sharpshooter, I might correct all the cyl with a standard IOL, but with the expectation they will need touch-ups.

A good place to start is their topo and their ...

I think there are a couple ways to look at this, and part of it depends on what your/the patient's goals are (crispest vision at post-op month 1 vs. better uncorrected vision 30 years from now) and what lens you are implanting (monofocal vs multifocal, the latter being less likely to tolerate higher...

I agree with Dr. @Dr. First Last. I think leaving a small amount of WTR astigmatism is prudent. A discussion with the patient on what to expect and the possibility of needing a future astigmatism correction as a possibility.