Payback Period

It may be that there are others who are confident enough to read your two short formulas and know exactly what you're asking for. I'm not one of them. And I notice you've had at least 10 other views with no response.

I suspect most of us would feel more able to help by seeing the full context here. That is, if you were to share either the actual workbook, or a meaningful mockup that faithfully--without revealing any confidential or private info--reflects your actual.

Otherwise, you're kind of asking us to guess what you're doing and why. Help us help you: give us a bit more to work with. We don't even know what's in cells C3, D34, etc. That's just a small reason why seeing the workbook can help grasp your situation and deal with your question in context.