I have found it is much easier to control the amount of flour by setting the amount of wheat in the auto-production panel. I have 90 wheat farms and six mills but I control the amount of flour by changing the number of wheat I want harvested. You have to keep in mind that by setting the wheat to two, for instance, that only two townies can be harvesting wheat at one time. The wheat is immediately taken to a mill, leaving a spot for another townie to come harvest wheat. Of course, if they are far away, it can take them awhile to get there.... However, that way you do not need to manually turn your mills off and on. If your wheat starts to build up, turn down the number for wheat production. I have found that in a beginner town of 11 people all being fed on bread and fruit pies, setting wheat production to 2 or 3 is more than enough. Flour starting to take over the map? Set wheat production to 1 for awhile. That being said, I transition over to ham and eggs and cooked beef for my people's main munchies later on and only have bread and pies as backup food so I set wheat production to 0 at that point and just manually harvest wheat when I need it. It did seem to me from times I stayed on pies a long time (no iron
) that about 2 wheat production for every 15 ish people is a good guideline - as long as you have enough wheat farms and enough mills that neither of those are a choke point. There should ALWAYS be enough wheat for harvest and ALWAYS enough mills that they can take the wheat right to a mill after it is harvested, otherwise, using the production panel won't work as well.
Anyway, that is just how I preferred to go about it - just seems a lot easier to turn one number up and down than to have to turn each mill on or off all the time.