I have a different answer for you. I think your 312 is a mounted planter. Anyhow this came up today when we were adjusting for beans. In 30 inch rows, a distance of 17-1/2 feet for eac row is 1-1000 of an acres. So calulate the distance your drive wheel travels in one revolution, and then turn the drive wheel an amount equal to 17-1/2 feet. Catch the kernals in a can and see how many you have. Adjust accordingly. Looks to me like you need one to drop about every 7-7-1/2 inches. Allow a little for misses and drive wheel slip. Have fun planting