question about fixing hp5

hello,

to fix a roll of hp5, the ilford data-sheet on their website says that
i should use a 1+4 dilution of ilford rapid fixer, but i have a 1+9
dilution already mixed up; i'm wondering if i could just double the
fixing time and use the 1+9 instead of the 1+4 (as the 'strength' of
the 1+9 should be half that of the 1+4). would that work?

any help is greatly appreciated. thanks,
{pd}