So I am one those "centering freaks" and the process I use is to do a hi res scan of the card and then I open up the scan in Picassa so I can rotate left - right etc.
Then I use my trusty JRuler which allows me to determine the number of pixels of white space on each border S/S & T/B
From there it's just divide the number of pixels on one side by the total number of pixels on both sides to get a percentage.
JRuler is a free tool that can be found at :
http://www.spadixbd.com/freetools/jruler.htm
hope this helps..............