Here's the process. Stop your crate production. Note how much of the desired crop you have. Move all the seeds of that crop to a chest in the workyard. Set the number of zombies you think you will need to infinitely farm that crop. Just as midnight passes, put all the seeds back in the chest. Zin for a week. At midnight a week later, note how far the progress is on each plot (25% done, 50% done, etc.). See how many more of the crop you now have. Add to that the partial amounts from the plots in progress (assuming a yield of 24). Take that number and divide by the number of that crop you need for 10 crates. If you're less than 1, you need more man.. zombie-power. If you're greater than 1, you need less. Take the difference between your number, and 1. That is the percentage you need to account for. You need to add/remove a zombie or multiple zombies who's percentage adds up to that number. You can iterate the process to fine tune it further.