That is true John, but I have been making MP3 disks for my car stereo for 6 years or so and have stored up to 250 mp3s at 128kbs. Gurney has stated that it seems to only want to accept 96 to 98 files which is more likely a file size issue... maybe the bit rate on the mp3s are large (256kbs?!?) but it seems that it's more a problem of data blocks being used instead of linear sequential storage where even if the data is 5 bits long it's still stored as 8 bits.

Some algorithms can overcome this and even burning software caters for the extra abundant space so this I would rule out of the equation with this problem. It's most likely that in the finalisation process it needs around 1MB to store disk information such as TOC and the 'rounding' process of burners saying that you are trying to store 699MB when really its 699,498KB.