Sorry for the slow reply, somehow this escaped my vCampus filter. As to your question, can you tell me how you are populating the AF attribute with the downtime in minutes? What type of application are you using to generate this data?
Are you familar with PI Event Frames? It is perfectly suited to track events such as downtimes and the AFSDK provides a rich set of methods for you to interact with these event frames.
François, did Steve's suggestion brought you any closer to your solution?