Grade it as if there was no damage.
Take perhaps 60% or more off the value for grade.
Unfortunately, the gouge in the neck amounts to grievous bodily harm
"AU58 details" is at least the best approach in which to grade it.
How much it should be devalued is a personally subjective thing, in the mind of a prospective buyer.
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The question for all of us to answer is:
'How much would you devalue it?'
60% is my opinion only - what is the opinion of each of us?