What is the actual problem here? Put your $15 of gas in... record your mileage... enter the gallons added and the miles traveled and mark it "partial tank." If you burn through a full tank and then only add a couple gallons and want to put a fuel up on fuelly, there's literally no way to give you an accurate mileage number for that tank alone. The information you have (say, 300 miles and 3 gallons) is insufficient to give you a number that makes any sense. It must be averaged with other tanks to give you any remotely accurate number. What you are asking for is impossible and fuelly has the best possible way to do it already.
If you're really that broken up about it, just don't click the "partial tank" and see what it gives you. But you can't input bogus partial-tank numbers and expect an accurate single-tank MPG rating.
I really can't understand why so many have difficulty understanding how MPG is worked out! Start with a tank, drive X miles, refill the tank and hey presto, you can calculate the exact MPG using the ACTUAL miles driven and the ACTUAL fuel used - simples!
If cost of fuel is an issue (which I can understand), then fill your tank once. Then you have your trip whereever and refill it immediately out of your £20 or however much it takes. That way, you will be paying for exactly the amount you used rather than a token £10 or £20. Even if you only stick a few quid in to top the tank off, fuelly can then calc the MPG. Although you have only 'partially' filled the tank, it is NOT the same as say running the tank low and only topping up a little and not reaching full.
Basically, Full to Empty to Full is good for fuelly. Full to partial to Full is good for fuelly. However, Partial to partial to partial to partial means you can't calculate anything at all as you have no idea how many miles pertain to what amount of fuel. (other than MPD - miles per $!)