The Daily usage savings percent are reported via UMA for the amount of data that is saved such as:
Net.DailyContentSavingPercent_DataReductionProxyEnabled*

However, these percents are recorded with the assumption that the savings cannot be negative. Thus, there should be a corresponding UMA to record when it is inflated. Because UMA percents need to be between 0 and 100, these percents should reflect the savings the user would have gotten by navigating without DataReductionProxy over using the DataReductionProxy.