On the [Diceware][1] page is this little gem:

> For extra security without adding another word, insert one special character or digit chosen at random into your passphrase...  Inserting a letter at random adds about 10 bits of entropy.

By that statement, does that mean adding a random character to a random position in a Diceware word adds 10 bits to each word?

What would be the entropy per Diceware word if random printable ASCII character is used instead?

How exactly can this be calculated?

  [1]: http://world.std.com/~reinhold/diceware.html