how the field "logical data size" is calculated from the CLI

I'm looking for the external API that either shows this data or the datasets and algorithm  that gives similar information as the command 'volume efficiency show -fields logical-data-size'.  I believe it is the same as the 'effectiveUsed' object from the powershell cmdlet 'get-ncefficiency' but I am not certain since the CLI is rounding it's results.  Any help on this would be appreciated.