For O1, I usually temper twice for two hours at 400 for ~62 HRC.
I have a high-end toaster oven in my shop that is really accurate and has a timer and stuff on it. It works like a charm and uses very little energy compared to my gas kitchen oven. My blades are usually a brownish yellow when they come out. I cool them in still air, then put them back in for another cycle.
Question for Mike: does it matter how the steel cools between tempering cycles? I've seen it recommended to water cool 52100 between cycles, which I've had good results doing, but don't understand this metallurgically. Is it like putting cold water on spaghetti noodles? :)