Ours is 10% and that's what the vet supplies for lambing too. It does sting the poor wee mites, though! I have seen farmers dilute a strong iodine to around 2.5% but since I have found the 10% does the job I have been loathe to change.
I prefer to spray, BH dips. Either way the important thing is to cover the whole cord, from belly button to the torn end, to stop infection getting in.