Don't see how one could electrically calibrate it. When the float/arm is all the way down it's all the way down. Supposedly that's when the fuel gauge starts flashing. If you add some external resistance, series or parallel, the position of the arm is still going to control everything. When they tweaked the shape of the tank in '06 they probably didn't tweak/change the fuel gauge sender. The '06 tank has more fuel down low near the bottom so there's more gas left when the gauge starts flashing.
I know I am replying to an old post here, but it made me think that perhaps there is a way to externally trim the fuel display. According to the even older post by ionbeam, the fuel sensor should be presenting 20 ohms resistance at full and 140 ohms when empty.
It would be quite easy to place a fixed resistance in parallel with the sending unit that would reduce the total resistance seen by the ECU, which would result in a higher gauge display at a given fuel level. Because the resistance at full tank is lower than at empty tank, the parallel resistance would have a proportionally larger impact at empty than at full.
The formula for parallel path resistances is : R1 x R2 / R1 + R2.
You can also cheat and just use this online calculator:
https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/
So, as an example if we put a 3k ohm resistor in parallel with the sensor, at fully empty the ECU would see an equivalent resistance of 133.76 ohms (4.5% lower) and at full tank it would be 19.87 (only 0.7% lower).
There are a couple of things that would need to be determined before trying this. One is to experimentally determine the actual sensor resistance when the fuel gauge goes to reserve mode. It is probable that this happens at something other than at the sensors maximum resistance, otherwise even a small variation in the sensor resistance could result in the display never switching to reserve mode. You should also be able to determine how much fuel is left in the tank when the sensor bottoms out and hits its maximum resistance value of 140 ohms.
Should be able to get this data relatively easily just by fully emptying the tank and then making iterative resistance checks as measured amounts of fuel are added with the tank and bike level.