I would say 90% is the ideal percentage. If your game is not meeting the optimum resolution + framerate the majority of the time, then it makes very little sense to implement a DR fixture in your engine. The purpose of DR is to curtail drops under the largest and most intense scenes in your game (in essence, when the shit hits the fan), you want to maintain your framerate even in these scenes.
The way Witcher 3 does it is on the flipside, which makes no sense for DR. It runs 900p 99.5% of the time, then what's the point of DR in that game at all. If anything, it should be doing dynamic resolution from 900p to 720P as I'm sure there will be drops below 30fps on the xbone version at 900p.