Moyer Marine is "the" name in A4s. See Atomic 4 Fuel Consumption - Moyer Marine Atomic 4 Community
where they discuss less than one gallon per hour.
Having a clean hull, clean correct prop, and not pushing top speed will all make big differences for fuel consumption in any boat.
I'd also suggest taking a shorter sea trial, maybe two hours or a half day, to shake down any possible problems before running off 80 miles. Measuring fuel consumption is something you could do during that time as well.