When to pull off some of the heat is hard to say. It would depend on the BTU output of your burner or stovetop and the amount of water you are steeping in among other things I'm sure. But I would say start turning it down about 10 degrees prior to your target temperature, and watch it closely creep up. If you have a good thermometer and get the temperature rising in a very slow controlled maner close to your target temp, you should be able to stop from exceeding that temp by more that about 2 degrees or so.
Another idea, which I had never even considered til I started AG brewing, is using infusion. This would be where you raise the water to a certain temperature, mix with a grains at a about 75-80 F, and the final temp stabalizes at your target. It works great for AG mashes. A good rule of thumb is: for a quart of water to a pound of grain, 16-18 F temperature loss. From there you may have to do some math, or use a nice program like Promash, but hope it gives a place to start. Happy brewing.