I like the way that Slic3r does it - you set the min, the max, and it automatically figures out what you need. Plus there's an option to keep it at a given speed all the time.
Also, and this is important for M2 owners, at least earlier ones - please "blip" the fan to 100% and then throttle it down to whatever speed. I know that in my case, and in the case of several other owners, that the fan won't turn over at (much) less than 100% from a cold start. This might be an issue with later hardware revisions.