We like to run about 4-5 gal with just 10-34-0 with some zinc added in before it is delivered. I know of some guys who have went a fair bit higher and have had some germ loss due to all the extra salt in the 10-34-0. I also have known some that have went higher with no adverse results. With the cost of corn seed I kinda want every seed that I plant to have the best chance for coming out of the ground so we keep it under 5. No extra phos added later for us.