Hi Ivy037,
You should be able to do what you have described by writing the value to a file, and then performing the math on the next measured values. You can find examples of how to read and write to a file in the LabVIEW example finder (open LabVIEW, click on the Help menu, and then click 'Find Examples')
If your device supports it you can also do a self calibration in MAX.
Here is a useful document on calibration procedures:
http://ae.natinst.com/public.nsf/web/searchinternal/305db655c3c5c7628625710a005198f7?OpenDocument
Please let me know if you have any further questions!
-N
National Instruments
Staff Certification Engineer