Add a new car.
 

I added a new car on 6th April and entered the odo reading and an estimate of the fuel supplied by the dealer. On he 16th I fuelled up and entered it and the new odo reading as a fuel up. But Fuelly divided the mileage covered by the amount of the new fuel up and not the initial entry from the 6th.

How do I enter a new car supplied with fuel and get the correct first calculation?

Please try adding it again so I can understand/troubleshoot what's going on. Although...
Adding an estimated figure and expecting an accurate calculation isn't going to produce the results you're wanting.

Normally, I'd wait until YOU physically fill up the vehicle before adding it to Fuelly. Unless you know the exact mileage and exact quantity of fuel that was pumped by the Dealer.