The creator is Nick Schultz, and here is his eBay listing, if you are interested in getting one...
eBay Grademaster Card Centering Tool
(FYI: I do not know Nick; I bought mine from his eBay listing; Not promoting his item and not receiving gratuities. Just providing info on an item I thought other collectors might be interested in.)
Here is what you get...
The centering tool, instructions, and percentages card.
I love it. Really easy to use.
Here is a youtube video I found on using it. (this helped me to determine to buy one)
(not promoting this youtube site, Just the information he is providing)
Hope this helps anyone looking for a better way to determine the centering of your cards.
Thanks for reading.
Keep On Collecting !!