Hey, {{first_name | Founder}}!
Customer calls often end at the worst possible moment. You describe the idea, the person nods, and says, “Yeah, I would use that.” You leave thinking the problem is real.
Most of the time, you only learned that the idea sounds reasonable and the other person is polite. You did not collect proof. You collected manners.
Before you end the call, ask for one small action. If they lose hours fixing a spreadsheet, ask them to send it. If somebody else owns the problem, ask for an introduction. If the problem matters now, book the next call while you are both there. If they want the result soon, offer a small paid version you can deliver.
A compliment costs nothing. A file, an introduction, a calendar slot, or a payment does. That small commitment tells you more than another ten minutes of praise.
You do not need a dramatic commitment. You need something that would not happen if the problem were only “interesting.” Instead of ending with “Would you use this?”, ask for the spreadsheet.