I don't have an oscilloscope so I can't give you values.
But I can say is that it is not critical. When I had it all the way up I was told, when I asked, that the voice was "quite readable but the compression could be turned down a little".
So I don't think you risk over driving it to the point of making it unpleasant.
I adjust it to the point of having the power meter at about two third in average on normal talk.
Hope this helps.
73, John (VK2ETA)