In response to the Division by Zero question, I think the right answer is that slack is zero in that case.
Here’s why. Take one of the components, say spare money. As spare money tends to zero, scarcity diverges off to infinity. Therefore slack, which is 1/scarcity, tends to zero as well. So this formula is defined for all values of spare money except zero, and at that point the limit is defined. I would recommend simply filling in the gap.
In response to the Division by Zero question, I think the right answer is that slack is zero in that case.
Here’s why. Take one of the components, say spare money. As spare money tends to zero, scarcity diverges off to infinity. Therefore slack, which is 1/scarcity, tends to zero as well. So this formula is defined for all values of spare money except zero, and at that point the limit is defined. I would recommend simply filling in the gap.