I've attached a quick spreadsheet on some common air tanks to help producers calculate how much fertilizer to put in each air tank so it runs out at the same time. It also tells you how many acres per fill you'll get at your desired seeding and fertilizer rate. If your particular air tank isn't on the spreadsheet, just enter your tank volumes on the left column and your fertilizer and seeding rates in the yellow column and it will tell you below you're acres per fill and lbs/ac per treank. 

http://beyondagronomy.com//cmsFiles/documents/document4fa6972db2c6a.xlsx

Good luck out there!

Steve