Jon,
... Fox 35s on profiles most often need the fuel pickup (or inside end of uniflow vent tube) about 1/4" (6mm) higher than the center of the venturii. Some fluke about the engine is involved. You can come close to proper tank height for a specific sample height by a ground test, too.
Start the engine and set it clearly rich. Roll the fuselage outboard tip vertically down, keeping the fuse centerline horizontal. Roll the wings to 45º cockpit-up and note or tach for RPM change. Roll it 45º cockpit-down and, again, note or tach RPM change. The 45º angle isn't critical, but try for the same angle both ways from wings vertical, and keep the fuselage centerline horizontal.
If the engine goes richer in one "45º" rolled attitude, the tank is relatively too high in that direction -- you are getting some gravity feed added to the venturii fuel draw. Cross check: both "rolled" attitudes should cause an RPM change from fuse horizontal/wings vertical.
What this does is simulate, to a useful degree, the load angles on the fuel in flight. Gravity and "centrifugal" force both act then, and we can't simulate the quantity of CF on the ground. We CAN simulate an angle for the "resultant" of those two forces, though.
Of course, after finding which way the tank is too high, point the nose straight down, with outboard wingtip a bit high to starve off the engine. Shift the tank to lower it from the "too-high" side, and run the check again. You should find a tank height with very little, ideally NO, RPM change between the rolled positions. That should be very close. If the model flies nose high or low, you will notice a slight run difference. This is more evident with uniflow vented tanks.
BTW -- Fox iron on steel engines do not like synthetic oil at all. All synthetics are detergent, and these metals need to be saturated with hot castor residues to run long and well. Detergents make shiny metal, where the Foxes need to show the dark residues on the piston.
Best of luck -- it should be a very enjoyable combo!
-Paul
Paul