"Jack
Something I've put in a request for (but it's never simple to implement in the sim being built in Java) is to have the fuel display as a decimal number when under 10L. That way you should generally be able to work out if there's enough for another lap or not.
There's even some guesswork involved if you know exactly how much is in the tank. Your pit box is sometimes quite a way past the line, and on occasion I've calculated another lap in my head only to forget I'm going about 400m past the line too, then run out between the line and my box. :P
Out of interest with that extra 400m or so, does it calculate it as full throttle normal fuel usage for 400m, or less fuel consumption considering you're going slower in the pitlane?