It's something of a mystery why there is less payoff today but let's skip to why you need 15%.
Let's say the biggest file on the E drive is 15% of the volume. To defragment that file the MS Defrag program may need that much space to copy the file to. While some could argue a smarter defrag program could do with just spare cluster then we get to discuss why MS's defrag isn't so smart?
So why not forget about it and enjoy your machine?
When I try to do a disk defragment I get a warning that my E drive only has 12% free space, and I need 15% to do a defrag. Then it asks if I would like to continue. I have tried several times, but always cancel before running the defrag. Do I ahve anything to worry about, or should I just go ahead and run it?