Rule of thumb is you want enough fuel for taxiing to position, takeoff, fly for 10 minutes, land, and then taxi back. You also want a few minutes extra in case a gust of wind hits when you are trying to land and you want to go around again. That being said to achieve this, an 8 oz. tank is considered standard for a 40 size engine and a 10 oz. tank for a 46 size engine. Since you have an 8 oz. tank with a 46 size engine I would be timing my flight time(starting at takeoff) for 8 minutes and then throttle down to land. When I get a plane that comes with an 8 oz. tank but a 46 is going on the front, I change the tank for a 10 0z. Running the engine on the ground at full throttle until the engine quits won't tell you running time. I have seen it tried and the flight time is always much shorter than the run time on the ground resulting in a surprized pilots looks when the engine has run out of fuel and suddenly quits. The plane flying through the air allows the engine to speed up, more rpms, more fuel being burned. [8D]