Hi Tim,
  I tried this method (thought I was smart until lots of folk said they did this!). It certainly got me very close to the correct tank height. It still needs a flight trial or two to get it exactly right, but its pretty close, even good enough if you are not too fussy!

Regards,

Andrew.

Take Leonard Nieuman's advice and shoot for 10 foot bottoms.  Then there will be some cushion and you can lower them when you get consistent.  If you can repeat 10 foot bottoms then it is time to go lower.  Saves on airplanes.

Lou Crane has several times posted a bench adjustment method he uses. 

Tim,

Sorry for the  delay...

My method that Jim T referred to is this:

Since the model is under loads from gravity (always 1 g straight down), AND "centrifugal" forces from flying in a circle...

...the fuel 'feels' a "local gravity" direction that acts the same as the two basic loads combined into one force. Most CL sport/stunt models fly at about 3 g, centrifugal.

The local gravity direction, then, is out ~3 units and down 1 unit. This is true flying upright AND inverted. Gravity doesn't reverse just because the model is going the other way.

To simulate the direction of that local gravity for both upright and inverted flight:-

Start the engine. Hold the fuselage centerline level during all these tests, except fuel shutoff.

Roll the model so the wings are vertical, outboard tip down.

Set a comfortable  needle that you can hear a change to richer or leaner. (Tach things if you can.)

Roll the  model to about 45° pilot's head up. Note and mixture or RPM change from wings vertical.

Roll it to the other 45° and listen or tach RPM.

If you've kept the fuselage centerline level, you'll probably note an RPM difference. Lock in your mind which way it went richer, or leaner.

Shut off the engine: Nose straight down, outboard wingtip up, and a finger over the fuel tank air vent will stop the noise in a hurry.

The tank is too "high" regarding the rolled attitude where the engine ran richer. That proves itself because it should have also gone leaner at the "other 45°."

Shift the tank accordingly, down away from the richer side, or vice versa. Check it again. After two or three such check runs, you should have practically identical RPM at both 45° rolled  attitudes. The tank height should be safe for reasonable flying. If it needs further tweaking, you can only determine that by observing several flights.

Yes, this only deals with upright and inverted level flight conditions directly, but the maneuvers are also "insides" and "outsides." That is, another g force is added to the gravity and level flight centrifugal. That other force still starts around the level flight middle, and should range with the tightness of turn equally both sides from neutral. Any height imbalance will definitely get worse under maneuvering g...