85% seems pretty good to me. I’d think that if you got to 90%, you wouldn’t be challenging yourself enough actually.
There’s such a thing as the 80/20 rule, which applies well to lots of things in life. So, it could apply here, if you want to give it a Google.
I personally have taken some advice that I’ve found on other forum threads, and use the number of ghost reviews I have as my guide for whether to add lessons.
it seems to me they start lower and gradually rise as i get better at them (then i get them wrong again at seasoned 3 lol)
(Only grammar)
Mine goes like this hehe. Keigo most likely dragged my N4 down but I’ve been getting better at it lately
I think you’re right, I’ll consider changing all my N5 reviews back to fill-in
honestly, 85% is good enough. I would only consider slowing down if you were below 70%. Having more items in rotation for your reviews kinda simulates the real world where any given situation might mandate a different grammar point. Being able to sift through everything you know to find an applicable one is a useful skill imo.
comparison is the worst thing to do when learning a language imo. cuz theres always people out there who pick it up quickly, say after a year or 2. But people i know irl who speak japanese studied for over 5 years before “fluency”, and are still studying now since you never really stop learning a language
Mine get worse as I progress through, despite thousands of reviews per rank 
Only grammar though, not started vocab reviews here.
Big big caveat on mine. As far as memory serves, I reset around the middle of N3 and then speedran back to the end of N3. I then took a slower pace for N2 (hence why the accuracy goes up) and part of N1. This was all manual input style reviews. I think I got rid of N5 and part of my N4 reviews at some point hence the weird numbers. I then stopped using the Bunpro SRS although still used it as my main reference. I then reset N1 a month or two ago and have been speedrunning using the reading style reviews in preparation for the N1 next month.
Note that N5-N3 was effectively revision. For N2 I would add things mostly manually based on if I had seen them before or could understand them without much effort. I also enjoy researching grammar probably more than the average learner and I believe that helped everything stick a bit more easily. Also lots of reading.
Grammar is definitely a slower road than vocab or kanji so I recommend just doing what is comfortable and going with the flow. Maybe invest a few months where you focus just on grammar around the end of N4 just to get the basics down tightly. No need to worry about the stats of others or speed as it is clear that so many other factors go into these things. I would also say even with high retention Bunpro has value in showing you how grammar is typically used. The example sentences are really good for this, especially for higher level grammar where knowing where grammar is typically used is king. 85% is pretty good I think!
