Don't you want the center of the tank in line with the spray bar, so you get the same fuel mixture when the plane is upright or inverted? [/quote]

You won't get the same fuel mixture, it will be leaner when upside down. When right side up the clunk is at the bottom and vent at the top, there is no restriction to the vent. Imagine pressure as inch's of fuel. The clunk is at the bottom and the spray bar is at the center lets use a round number for muffler pressure of 10 inchs of fuel, that is a bit low but it is very round. Lets pretend the tank is 4" deep from top to bottom. So when the plane is upright the tank pressure is the same as the muffler pressure because of the unrestricted vent and the fuel pressure is 10"of muffler pressure plus the 4" of fuel on the clunk minus the 2 inches from the spray bar to the clunk which comes to 12". Now when upside down the vent is at the bottom and is now restricted by the fuel in the tank, so the tank pressure is now the 10" of muffler pressure minus the 4" of fuel in the tank or 8". So the fuel pressure is 8" + 4" - 2" = 10" so the engine will run slightly leaner because the fuel pressure is lower. In this case the best location would be 1" below the spray bar. Then when upright the pressure would be 10" of tank pressure + 4" from top of fuel to clunk - 3" from clunk or 11". When upside down the pressure would be 8" + 4" - 1" or 11" which is the same pressure. Actually the rule of thumb should be that 1/4 of the tank should be above the spraybar, but 1/2" is close enough for most installations. In fact even centered is often not noticeable.