I'm not sure if the following observation scales down the problem, but may be 
it's of some help.

One of the problems here is finding a (nice) partitioning of bucket sums that 
are equal or close to the mean bucketsum. Looking at the two presented examples 
I found the following:

We have
  bm : mean bucket sum
  nb : # buckets

then you may say
   (,>:) <. bm
are two possible sums very close to bm.

Then we have to find a and b in the following equation:

(+/m) = +/ (a, b) * (,>:) <. bm

Finding a and b:
   b=. nb * (-<.) x: bm

and a follows from

   a =. nb - b

For the low scale example we have:

nb=. 4

    ]bm=.nb%~+/m
123.25

    ]'a b'=.(,~nb&-) nb * (-<.) x: bm
3 1

    (+/m) = +/ (a, b) * (,>:) <. bm
1

For the large scale ex. we have:
nb=.20
bm=.5273.4

    ]'a b'=.(,~nb&-) nb * (-<.) x: bm
12 8

Giving bucket sums:

12 8 # (,>:)<. bm


I'm not sure of course if this particular partitioning (a,b) of integer bucket 
sums is possible.

-- 
Met vriendelijke groet,
=@@i

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