I currently have a 95% target retention rate. It is still a bit too high from when I brought it down from 97%. When 90% be too dramatic of a reduction? Or should I split the difference and go with 92? I'm just trying to get an idea of exactly how much less my reviews will be.

How long have you been on 95%? It takes a good while to propagate.

The differences should be rather large, so you can try 92%, but you'll have to wait a good while to see what the difference really is. Or you can try 90%. It's not going to kill you; it just might be a little more uncomfortable forgetting more of your reviews. It just depends on personal preference, how many of your reviews you want to get right.