concrete shed base - how level does it have to be?

[bcm454]

Hi all, I am building a concrete shed base 15 sq metres and as I have used large lumps of concrete rubble to fill in a bit of a dip, I have depths of up to 25-30cm (10-12") that would need to be poured with concrete, in places. In most places, I have filled between the hardcore with 20mm stones to allow a 15cm concrete slab over the top. I am undecided whether leaving a few deeper pockets between hardcore would let concrete pour in and give a stronger result and a better key to the ground or whether I should fill all gaps with stones/shingle so that the concrete has an even and level bed to sit on, perhaps avoiding stress points and reduce the risk of cracking in the future? The sides are either wood shuttering or brick retaining wall. I am not proposing to use a damp membrane.
Does anyone know which would be the better approach?
Many thanks
Brian

 

[Thermo]

wont make a difference in terms of strength. its a 6 inch shed base and its only going to be loaded with a shed