It took my awhile but I was able to just memorize which were which. I found playing a few rounds of online sudoku really helped, I posted a thread on how to get the kindle to work on the websudoku site
here if you're interested.
I also found associating the numbers and letters with some type of phrase helped, for example the number 7 is alt+U so "7 Up".
I really don't see why amazon didn't just do this for us since there already is silkscreened print on the device.