K2's reporting is all built on top of SSRS, so it is just a matter of writing a report to display the data and do the calculations you require. If you haven't wrote a SSRS report for K2 before, it is straight Reporting Services report creation and there is a ton of information the web that can help you through the details. The only K2 specific component is that you will need to use the K2 ADO.NET provider for SSRS to be able to access smart objects which will give you access to the metrics you are needing to report on.

For K2 blackpearl, you will open your process in Visual Studio and in the upper right corner of your design canvas there is an associations button. It will look like a small green flow chart icon. Click this button, and you will be presented with a window with an option toward the bottom called "Create Workflow Reporting Smart Objects." It will be checked, but you will need to click settings to the left and enable a more granular level of reporting. You can even set which activities you wish to report on. Once you are happy with your selections, click OK and Finish to get out of the wizard. Save your process and redeploy. You have to redeploy to force the creation of the new Smart Objects.

The purpose of this is to make sure that the necessary reporting Smart Objects have been enabled. By default, K2 only creates process level reporting smart objects in order not to clutter up the environment with unneeded items. 

Whenever I teach a K2 blackpearl 101 class, I always suggest to students who are getting ready to customize their first K2 report to use the basic report builder in K2 workspace to select the data they are going to want to report upon, save the basic report and publish it to the Reporting Server. Then, navigate to the Reports website and retrieve the RDL file. Finally, add that RDL file to your new Reporting project in Visual Studio. What this does for you is that the basic tool has wrote the query for you. It may not be EXACTLY what you need, but it is a great place to get an example to use as a starting point. It will familarize you with the Smart Object naming structure and how exactly to use them in a SQL query.

Finally, there is a Report Viewer web part you can deploy into your MOSS environment that is available from Microsoft. Once you have your SSRS report created, simply add this web part to the page where you want to make this report visible and configure it to point to the report on your SSRS server. Your users will never have to leave MOSS and will never be the wiser as to where the report really lives. 

 Hope this helps!