You should have some usable free space now then. :) It's tricky though because of the way btrfs works the free space estimates are just that, estimates. Generally I would try to keep it under 90% reported utilization (by the regular 'df' command) at all time and at about 95% utilization start really deleting things.
I'm using it on a 4 disk Raid1 array with about 1 TB and it's been pretty stable. I've taken it down briefly to about 30 GB free space with no problems.
Even though you are using 'single' you can also use 'scrub' to check data integrity (although you will not be able to fix most errors with it since you are not using raid1).
btrfs scrub start /
You can check status with:
btrfs scrub status /
I also have an entry in my crontab file to do a automatic weekly scrub on Sunday at 4am:
0 4 * * 0 /usr/bin/btrfs scrub start /