Technically, yes (for "50:50") - but, no matter what sort of 'average' it was, there would (unless all cases were identical!) be at least some cases in which the 'needs' were on the side of the average which rendered inadequate a size of cable that would be OK for the 'average need'.
Well that would be about 6kw 50m run or 10kw about 25m run. I suppose in reality the max loaa would be the nearest lower mcb rather than cable capacity.
Average for a shed might be a couple of lights and a couple of double power sockets. 6mm cable might be quite generous......
I suppose there is not really such thing as 'average' as each circumstance is unique, however I guess electricians probably get a feel for what cable is more common over years of working.
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