How are tapes declared full

[Angus Macdonald]

Doing some maintenance, I notice that most of my LTO2 storage pool tapes are 
designated as full, although their % utilised is sometimes less that 60%. The 
estimated capacities also vary from 320GB to over 600GB. I'm not actually 
having any problems but how does TSM decide the estimated capacity of a tape 
and whether or not it is full?