Thanks for that, I was aware of block sizes, but didn't think of the other two. (my plan was to use the results of this test and 'scale' by the report of df).
I've compared this one with du, and the runtimes are not dissimilar (174m compared to 162 for the script).
I'll have a look at replacing the find with a du, since that would seem to make some sense, and compare the two.
Cheers.
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It's not pessimism if there is a worse option, it's not paranoia when they are and it's not cynicism when you're right.