Bissy,
I can absolutely confirm that the Army does 1 in 5 the same as the RAF (as indeed does 3 CDO Bde), the so called rule 5. I suspect you may be confusing it with a ratio of 1 to 4 which is the same as 1 in 5. Feel free to look at the latest Land Cts plot or indeed any Land Cts plot from the last umpteen years if you don't believe me. The RN do indeed have a quicker rotation in their Harmony rules although I was under the impression it was 1 to 3 (1 in 4) rather than 1 in 3.