Legally, you can apply 90 days before.
However, in many cases, the process is moving so fast that people are getting interviews in 70-80 days after filing. What then happens is that even if the interview went well, the IO just keeps the file on hold because the #days of residency is still not complete. Sometimes, the IO will approve the case right when the 3 or 5 years are complete, sometimes they will forget and people will keep waiting for an extra month for approval.
The other factor is that USCIS is usually not forgiving with respect to early submissions. So if you miscalculate by 1 day, or even if you calculate right, and USCIS mis-calculates - they deem the filing early and can just reject it.
So the 85 day advice is to protect against CIS miscalculation, and the 70-80 day advice is to protect against your case moving too fast to be approved at the time of interview.
Re file moving too fast - you can check the processing times of your DO by reading about other's experiences ... not all DOs have the same speed. Anyway, by the time, you are ready to file --- a lot could have changed in the next 8-10 months, so check the information then.
[Sorry I probably mixed up he/she in the previous posting, since I did not check.]