There are 20 places to put the random character, and there are roughly 100 possible values, for a total of 2000 possibilities. Thus, assuming someone has knowle

There are 20 places to put the random character, and there are roughly 100 possible values, for a total of 2000 possibilities. Thus, assuming someone has knowledge of you using this trick, it corresponds to adding 11 bits of entropy. There are other similar tricks you could have been using instead, so there may be a few additional bits of security if they know you use some trick but not which one.