When I'm putting together fertilizer blends I'm usually working through a few spreadsheets to calculate desired nutrients, product rates, densities, total volumes and cost per acre. I hired a university student taking linear algebra to come up with a solver in excel to do all the calculations I need in a few small steps. I gave him some money towards tuition and he wored with me to develop this spreadsheet.

You can find it here: http://beyondagronomy.com//cmsFiles/documents/document58ea46c177afa.xlsm

Steps to make it work

Enter values in cells coloured yellow. 
Enter acres (left), desired nutrients (top right), product density, product analysis, cost per tonne (bottom right)
Press Control R and then hit okay. 
Voila! Actual nutrient blend, product rates, volumes and cost marked in green.

If you have any questions I'll do my best to help. SL