Oops, 5% is a bit low! Need to keep 15% free-space on all partitions really for defrag to work properly. More where the paging file needs to grow and there are a lot of temporary files which accumulate during a session.
I just use the 1st Logical drive within the Extended partition for a dual boot.
It is aways handy to have a dual boot if the primary OS won't boot for some reason, or to have a KNOPPIX CD handy.