Why do I get an error message when I use “compose your own” fertiliser saying “Total N entered does not match calculated total”?

Why do I get an error message when I use “compose your own” fertiliser saying “Total N entered does not match calculated total”?

The % of total N entered in the top row must be the sum of %N entered for nitrate, ammonium, and urea (e.g. if 30% N comes equally from ammonium & urea, it’s 15/0/15% ). Please correct the values.