Perhaps this would help if there is a way to request by specific BT IDs?
https://docs.appdynamics.com/display/PRO41/Use+the+AppDynamics+REST+API#UsetheAppDynamicsRESTAPI-request-snapshotsRetrievetransactionsnapshotsforaBusinessTransactionforatimerange
/controller/rest/applications/<AppID>/request-snapshots?time-range-type=BEFORE_NOW&duration-in-mins=<time>&need-props=true
Also, would the percentages just be: (specific BT's timeTakenInMilliSecs/all BT's timeTakenInMilliSecs)?