Hi claverty,
if one third of an sector is missing you should still be okay. If at least one third of an sector is present then the energy of this sector is there and I wouldn't worry too much about it. Some schools recommend mirrors to give the illusion of more space but I would not advise mirrors in the bedroom (too yang, disturbs sleep if you can see yourself from the bed). On the other hand - I tried this method in my own home and mirrors don't work to bring in the energy of a missing sector.
So, if only one third of this sector is missing you should be fine.
All the best,
Astrid
