Not quite. Lets look at the units, you are trying to work out litres/100km. litres is easy, you used 44L. Now how many lots of 100km's did you do? 420/100 = 4.2.
Therefore, our equation is: 44L / 4.2 = 10.47L/100km
You have worked out km's/L ie 420/44 = 9.5, ie, how many km's can I drive with 1L of fuel. To then get L/100km we can invert (swap km/L to L/km) and multiply by 100 and get the same result as above, ie (44/420)*100 = (1/9.5)*100 = 0.1047*100 = 10.47L/100km, same as above.
Just think of the units, they tell the story no matter what quantity you are dealing with, whether it be psi or Nm, most of the time anyway! Off track a bit the common metric pressure is kpa or Bar, where these units give no meaning to the physical quantity, ie you have to remember what a kpa is, you can't work it out from first principles, which I think is why most people still work in psi for the unit of pressure. Its probably easier to remember metre's of head and convert to kpa ie a column of water 1m high produces a pressure of 10kpa or 0.01Bar. Sorry for the rant but it is one quantity I struggle to get a 'feel' for in the metric equivalent.
Anyway..... Still not too bad though, not much worse than the diesel, but I wonder what it would be like through the Simpson or similar compared to the diesel.
So even with the increase on fuel tax for you Brisbanites, you are still paying 50/44 = $1.13 per litre for fuel? Its at least 5c more than that here in SA.