I’m not quite sure what you mean by “learn to suggest”. The logic I explained above for how the calculations work does not change from month-to-month. What does change, however, is the period of the Envelope. A Monthly Envelope with a Due Date will work on a cycle, so every month after the Due Date, it’ll reset its internal counter for how much you’ve Filled so far this month, and tell you that you need to Fill again – again, regardless of your current balance.
In other words, even if the calculation seems weird or silly this month, it’ll reset for next month. That being said, if you do the same actions that caused it to display a non-standard amount next month, it’ll show something non-standard again next month as well.
Whether or not you want to use a Due Date on your Envelope is totally up to you and how you budget. Because of how they’re designed, they’re best for recurring and mostly fixed value expenses, like Debt Payments or rent or things like that, rather than things like groceries or gas where the amount spent in a month will vary. But like many things in Goodbudget, the “right” way to do things is the way that works best for you and your budget.