If you build a spreadsheet with info they gave for N, you will have something like the one I attached, where the only you need to know is yield target.
Columns 1 and 2 have lbs/acre for the number of stage days and yield, and cumulative lbs/acre respectively.
When you do the pre side dress nitrogen soil test you get the N #/ac soil has, with this info and the table you could know approximately how many days you have to do the side dress without your crop be hungry.
In addition to that, each university has his rec, for your case is IA, see here what they said
How to know when to side dress corn
