This should be collected from monEngine and delta'd and put into SAPTOOLS database for you. If you use DBACockpit, it will display the data. If not, you may have to look at extracting/plotting the data out of SAPTOOLS. I would suggest looking at both monEngine and monThread.
For monEngine/monThread and most MDA tables, all you need to do is subtract sample #1 from sample #2....so just do the delta. Result should be extremely accurate as I do this all the time without issue.