Ritardando

In congregational music we generally gradually slow down a bit when ending a song. (I don't hear ritenuto--immediately slowing down--very often) Is there a pattern that generally sounds best? (e.g. making the penultimate measure 25% longer and the last one 50%, or lengthening the notes in the last couple measures by 10% over the note before, or maybe the fractions vary with the original tempo)

A quick googling found the term "ridardando," but it looked as though it not defined and is left entirely to the taste of the performers. Which is fine, but I wonder in practice if they generally converge on some patterns that sound better.