It has been a few years since we have used it but I made a spreadsheet that optimized the blend of DAP and Potash for a variable rate field were the minimum needs of each product would be applied to each grid (so no grid would get less than the recomended amount of P and K). One product was always over applied. The optimization was based on least cost or in otherwords most economical blend to meet the needs of the gridded field. We could only variable rate a single product, thus the need to optimize the mix so as to overapply as little as possible. Used it for a couple of years. Have been just doing blanket rate for the last few years. Getting variable rate set up on our Flexicoil cart is on the "to do" list but quite a ways down the list.

We probably are in a better situation now to benefit from variable rate fertilizer than when we were actually doing it. Where turkey litter has been spread for up to 13 years we have quite a few medium and high soil test levels. We just rented a bunch more ground and it all tests low to very low - pretty hard to get "too much" fertilizer on those kind of tests but our high testing soils we could cut back in places - no need to over apply there. 

You are welcome to a copy of it Darin if you wish.