I usually use Headus UVlayout. It lets you specify bleed spacing and does a much much better job of packing than max. I've sometimes got as much as 40% better coverage than max's packing. One of the best things about it is it lets you pack your shells in groups, which is really handy for hands with detached fingers and so…
maya has a built in packing function and so does UVLayout - they dont exactly tell you how much space you are wasting but they will pack the UV's together as efficiently as possible.
Yeah, but Maya's one sucks, and leaves loads of wasted space. UVLayout does actually give you a percentage, and has some nice features like keeping certain UV shells together/near each other so you don't end up with teeth/rivets/greebles scattered all over the map.