Foundations how what accuracy is needed on getting them level?

hammer some stakes into the ground roughly in the centre of your footing trench. then using a spirit level mark a pencil line to your desired height linking each stake as you go, then saw all of them off on the pencil lines. then fill your trench up to the top of the stakes. take your time and be as precise as you can. if your a few mill out dont worry the weight of the shed and contents will soon settle level . hope this helps