I know that this a late reply, but I was searching for a similar report and thought I'd add this response. It sounds like your manager is confusing server availability with service availability. In order to accurately manage service availability you need Server and Application Manager so that you can view the critical services (like email) in the cluster instead of the status of the individual clusters.
Hope that helps (and that you already found your way to SAM on your own!)
Even later reply - I agree with jbiggley on SAM being good for monitoring services availability.
However for quick and dirty up/down time measurements I have been using group availability charting - add the clustered servers into a group and choose best status rollup option. Now when you use the group availability chart resource (which you can schedule as an emailed report PDF) it will show 100% as long as all servers are up - network takes them offline and you'll see 0%. Quick and dirty.