The "amount needed to next harvest" was more useful back when the harvests were different lengths depending on the season. That meant that the amount of food required to get you from one harvest to the next could change drastically, especially when one harvest was in fall ( 5 days) and the other was in winter (15 days).
Now that all harvests are 7 days on Dwilight, that box really isn't as useful as it once was. You should be able to set a threshold to whatever your region requires for 7 days, and leave it.
...but sometimes you have a drought. I usually round up a bit.
