85% is the default level of Single IDAC Auto-Calibration.
For those designs with using 'Direct' sense clock, Set IDAC to drive baseline around 85%, will be good for avoiding flat-spot(For more details about flat-spot, refer to Section-18.104.22.168.2 in AN85951).
But it is recommended to use 'Auto' or 'PRS-Auto' sense clock in most designs. With using 'PRS-Auto' or 'Auto' clock, no matter where the baseline is, the flat-spot will be gone and won't bother you. And the system can get more EMI/EMC benefits with PRS sensing clock.
That is to say....if you choose PRS sense clock, you can tune IDAC to make the baseline lower, even around 15%. However, either too high or too low is not good for manufacture production, because you never know how the worst case of manufacturing consistence could be....
if you have to choose direct sense clock in some designs(e.g., long distance proximity applications), it is better to make sure baseline is always upper than flat spot area.