Just to make sure, so if you are in maintenance mode and you select option to add more disk space to / dir, it does not let you add anything?
Even if thats the case, 11 GB I would say is good enough. On average, from my experience, root may "shrink" just a little bit, so from 11 GB, after you upgrade, yours might show 9 or 9.5 GB free (my best educated guess).
I would still do this in the lab first, but I can tell you last time I did this, mine went from / free 7.8 GB in R81.10 to 6.6 free in R81.20 after upgrade.
Hope that helps.
Andy