# Unicity-distance of Playfair

Given the classical scheme Playfair, say, for 5*5 matrices, I am interested to determine its unicity-distance. How should one proceed to properly do that? I should appreciate it, if I could obtain a very terse recipe, listing the kind of computations such that I could effectively compute that value myself. (I am attempting to determine the unicity-distance of a suggested variant of Playfair.)