I`m planning to participate in the next round of tadoku, but wonder if anybody knows a good way to get a count of how many characters there is in a kindle ebook?

As kindle use locations and percentages to count progress I would think it`s difficult to record the correct number of read pages.

You could convert it to a text file with Calibre and then use an online character counter to get the whole length.

Then if the Kindle gives a percentage complete you could use it to estimate how many characters you've read.