How much did you pay last renewal? If you had a 50% NCB, then double that to get the gross premium. Now what is teh step back? is it back to 20% or 30%. If 20%, then discount the 100% by 20% and if 30, then by 30.
That will give you a very rough picture of where you stand.
However, as Jimbobp has stated, the premium would have gone up even without the claim, so it is a very rough picture.
Again, as pointed out, shop around.