It would also be beneficial to understand rollout to doctors once the app/update/whatever goes live. Do doctors need training before they use the tech? Is this via an online training module/vid (quicker option, give deadline to complete), or live video training session (would take longer, not as scalable)? Are they trained before go-live or after?
Usage uptake could be staggered based on these enablement factors and then you have payment terms on top of it.