Actually, whilst I've got the bit between my teeth...
The contents of my bottle of Ilford Rapid Fixer are:
Nary a word about proportions, of course, and there's nothing about it on the digitaltruth page, though this on Kodak F-24 fixer gives a clue.
Probably better to get the whole lot, developer and fixer chemicals, in one go, save on postage.
What larks, eh?