If you're RPM is already at the top of the green before you begin the flare it will definitely overspeed during the flare. So when you are in the steady state of an autorotation you should pull collective to keep it at a 100% and the ROD will be what you might expect as will as you're angle/distance. When you need a very agressive flare at the bottom, you might actually want your RPM lower to start the flare.