Aaaaand... now it dawns on me that with "buying a quest slot" you meant "buying a quest slot with quest items". Not with gems.

But the calculation is almost the same. Just add the cost for the additional slots to the quest items you need. In my first example it would be 8000+4000 and it would definitely be too late (that is if you don't want to spend gems).

Or, even easier, you just divide the slot cost by the days left and see if your estimated daily quest item revenue is above or below the result of the division.