If the bike is well tuned, it should give about 75 miles to the US gallon on "highway riding" which for a stock bike would be on a clear open road at about 85- 90 km/h.
That's about 31-32 kilometers per liter.
However, most Bullets are not optimally tuned, and often will yield about 80% of that figure. This is probably the most common fuel usage figure, around 60 miles per US gallon, or about 25-26 kilometers per liter.
So, for an average Bullet, you could expect probably around 200-250 miles to a tank full(11 liters) of fuel. On a perfectly tuned Fireball 535 like Chumma rides, he gets 262 miles on a 11 liter tankful before reserve, and he's riding on the highway at 70-75 mph and gets that amount of fuel economy.
So, to get a proper baseline, do a fuel economy run consisting of filling up to the top, then riding on an open road for at least an hour, and then fill up and see how much fuel it used. Then you can make changes and test periodically to see how the changes have affected the fuel economy.
One of the most directly beneficial ways to improve fuel economy is to increase the compression ratio, and tune to suit. Of course, that would come after you have removed the intake and exhaust flow restrictions, which you have already been working on. While it is commonly assumed that performance modification yields lower fuel economy, that is not always the case.