Yeah, before I topped it off for the trip, I had four fill ups (not sure of the mileage), and was between 1/2 and 3/4. That was as purchased from the dealership. Gauge read full, but it might not have been "FULL". I don't think my manual said how many miles I could get per tank of DEF.
As far as the "exact number", I came up with 7/8 as the gauge is between 3/4 and full.
I have nothing to compare my mileage to, since I have no experience other than this truck with DEF vehicles. I had only heard/read that I would use more DEF towing, and it doesn't seem to be the case.
Mike