Here's a map I made up for zone 9 that I thought would be a good place to set up a self contained silo and farming area (as I don't really care for the gold mines all that much).
Everything above the line utilises the top field, and everything below the line utilises the bottom field.
The numbers are the total travel times for either building to do it's thing in regards to their respective field.
Below the line, set up silos in a 36, 36, 44, 60, 60. Set up Farms in a 28, 44, 52, 52, 60. If you feel it's too far, optionally you can drop one of the 60 second silos and farms to keep it equal overall. Both groups have exactly equal total travel time (236 for all 5 of either type), so the field will never deplete.
Above the line, set up silos in a 52, 36, 36, 44. Set up Farms in a 28, 36, 44, 60. Equal walking times (total is 168 for all 4 of either type), the field will never deplete.
And that's about it.
1 storehouse, 2 fields, 9 farms, 9 silos, timings all match up for the fields to remain constant.
To ensure it's accuracy I let the field in the top section on my island drop to 162 wheat. It hovers up and down slightly as the different timings occur, but overall it has stayed steady for several weeks now.
As further options, there's a 60 and a 68 left above the line, you could pop a farm and silo in these respectively, but I wasn't happy for things to go over 60 seconds.
Similarly, there's a 68 and 76 left over below the line you could do the same with, but again, probably too far.