A nice idea........but one wonders how S3D will actually know how much filament is on the spool of filament loaded in the machine.
Full reels of filament are filled and sold by weight....not but length....so the actual length could vary by say 1 to 2%. That's something in the order of 3 to 6 m of filament for a 1Kg spool.
This is not a new problem. Ask anyone involved in the manufacture of electrical cables. To create a 500m length of multicore cable, they have to start with creating the individual cores in lengths of say 520m to ensure the made-up cable can be cut down to the desired length without losing any of the cores!
Apart from that, its like the "thrill of the race" estimating how much filament you have left and having to decide if there's enough to print the next piece. Lets face it....life can get too boring if you hand over too much to machines!!