Ok, my research indicates that 1% (or if you want 0%) on each partition is enough if you have a 40GB partition. The only ones which REALLY need to have space at all are those where root (or root owned programs) will write to:
/root (if in different partition to /)
The reason that this is reserved is so that, if these partitions run out of a space, there is still 5% available for root to write to temporarily so that he can delete/move files to free up space .
From the point of view of folders which are just holding user data (like /home) then you can safely reduce this to 1% or 0%.
Hope this helps the next person.