In your screenshot of the first map of grids you have 2 red circled areas. I'd estimate those areas to be approximately 100' x 200'. Would that be correct? Google earth view looks like growing crop with higher sandy areas that have died early?

So, in your zone testing, you would include each of those red circled areas as a separate sample point?

After sampling, testing, and recommending for that little 0.5A spot, how would you operate a spreader to address that small spot while spreading a 70-80 ft wide path while travelling straight on a N-S driving path at 8-12 mph?

Yes, a grid sample may miss your circled sandhills, but how many sample points would you have to take in that same field to be as effective as the grids you have shown, plus taking in the ones you suggest that grids have missed? 50 sample points perhaps for zone testing?