From time to time we get requests for assistance on using aggregations and building other types of summary reports. This report template provides a summary of the counts of computer/update value-pairs in each of the possible states in a WSUS Server: Unknown, NotInstalled, Downloaded, Failed, InstalledPendingReboot, and Installed, plus the Total Count. This particular report aggregates by WSUS Server and Update Source, but different aggregations can be produced by changing the fields contained in the report. For example, if you were only interested in reporting by WSUS Server, but did not care about Update Source, you can remove the Update Source column. If you wanted to aggregate by Update Classification, add that column. The key here is that this report uses the SUM aggregation to calculate the total number of computer/update pairs in each state.
To take this a step further and calculate percentages (which are not stored values in the WSUS database), export the data to excel and add/calculate percentage columns as needed.