Hi Dennis,
The rule of thumb, if you have not calculated the fuel consumption with your exact motors or with a flow scan for dual outboards with a full tank of gas, 1 mile per gallon. You can easily figure if you are getting more or less but I think that is a very good number.
So, using the rule of thirds (1/3 out, 1/3 back and 1/3 reserve) you good get approximately 87 miles of running at mid-range (3,800 rpm for Yamaha) with the rest for reserve.
What year and make are engines? We can look up the fuel consumption curve.
If you are trolling, that gas has to be fatored in as well.
The reserve is sacred. It helps on the days with nasty seas and winds.
It is hard to get exact numbers without a flow scan and each day is different with different conditions.
Best wishes,
Ed
I will be more than happy to help you calculate it this Summer.