8 oz is about 'normal' for a 40 size engine. You can make do with 6 oz. Fly for 5 minutes with your normal throttle setting and land, then pump out the fuel and see how much is left. Then increase the 5 minutes accordingly.

I fly a 3D profile on a 6 oz tank with a .50 size engine. I get 10 minute flights, but the tank is strapped to the side of the fuse so the remaining fuel level is visible.

As JohnBuckner said, sometimes too large a tank can present problems. I'm not sure why, maybe it has something to do with muffler pressure. I have had engine problems on two different models that were solved by decreasing the size of the thank.