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central WI | I would think that you should be able to figure out the average grade of the run. You don't list the lengths of each grade, but your tile program should be able to get you those lengths, or you could scale it on your map. I would figure out the average grade and then round down to the next 0.05% to be a little conservative. Technically, I suppose one could argue that the 'weakest link' or lowest grade would be the capacity of that main, however, the flow of water should push past that if you are truly draining significant water.
We always shoot for a 1/2" coefficient minimum, but sometimes have to settle for closer to a 3/8" depending on available grade. If your average grade comes out to be somewhere between 0.2% and 0.3% I would be happy with its ability to handle 12 acres at those drainage rates, assuming you are only going to be draining 12 acres total.
To figure out the grade on the old mains can be tricky. Typically, if you excavate a few sample areas, you can determine an average grade of the existing main. If there is significant variation in the topography it becomes a little harder to calculate. I would use the smallest average grade as the "controlling grade" of the main, and be conservative with that estimate as well.
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Average Grade vs Min Grade Tiling Drainage C/E
