I guess you can get an average number of nodes per transaction which called the subpath Length from the 'dynaTrace Self-Monitoring' system profile to use the 'PurePath Length' measure.
What time frame will you recommend ?? because i went with last 30 mins and Avg purepath length was 500. In spreadsheet i receive error as "The average PurePath length of 500 nodes is the default and also too high for production. Please make sure that this value is correct"