Even "40 in the shade" or "40 in the waterbag" can't quite drive the heat home as well.

Since I wouldn't have a clue about what the temperature is in Fahrenheit, 40 certainly *would* drive it home for me!

As for crazy conversions that break phrases, this is true in any system:-

Full 72-inches-es deep thy father lies...
Not quite the full 48 farthings...
And Winnie the Pooh lives in the 400 Rood Wood, a name which would trouble Dr Seuss!

Graham.