Alfredo,
I believe you're correct. In other words, with 350 gallons of fuel in the plane (not including 45 gallons not accessible in flight: a lot of dead weight to fly around with!), you could fly a maximum of 830nm at any altitude. If you want to go further than 830nm, then you'd have to fly below 12,000 ft to save fuel.
You can relate this to the engine flight chart: the minimum fuel consumption at 5,000 ft is 60 gallons/hr. At 25,000 ft, it is 95 gallons/hr.
I hope this helps.