The best way to prevent over seeding is to figure out how big an area you are seed and measure out the recommended amount of seed (like 2.5 lbs per quarter acre of maximum I believe) then set your seeder on its lowest setting and go back and forth several times over the area. You really want to stick with the recommended seeding rate on brassicas because they need a little room to grow good bulbs. If you over seed you will still get growth but it might be a little stunted