Anyone use model logs to communicate milestones?

Hello,

The scenario we have is that a purchase order is created with several, sometimes a hundred or more lines for a project. Payment of those lines occurs at milestones during those projects; for example arrival of items, a percentage of the project complete. So you see these milestones can be both tangible and subjective - no lines may be received at all or partially. Advanced payment terms allows a schedule to be made, but these dates can change, controlled by both AP and purchasing. Also, AP requires an audit and proof of authorization that a milestone is met and payment