I think this way.

How much N would you have lost to the crop if you would have put the N on before planting?  If you apply 100# on pre plant and loose 10# from pre plant to side dressing time than you can side dress 10 # less.

How are you applying the 28%? On top or in the ground.  On top you may loose some to the air.

How much rain will you get after the last opportunity for N application?  If side dressing is you last opportunity and you get excess rain will the corn have enough N for the season?  If the plant has to rob N from the stalk to fill the ear you will have weak lower stalks.  You will be able to just squeeze the bottom of the stalk, about 12 inches above the ground and it will just crush with ease.

I guess it just comes down to how close you want to cut you N expenses.  Some years you can get away with .8# or less per bu., other times not.

Example.  P dealer was give presentation and they grew 300 bu average yield on a whole pivot with 187# N application.  Hmmmmm.  We did have good August/early September rains, only watered 6 times instead of 9 times, cooler over all weather.  Yields here were 20-30 bu above average. 

I am going with 140 pre plant anhy. 60 # through pivot before silk and last about 20-30# at brown silk. Maybe 10-20# N in soil after last years corn.