This is strange, since Harvest should also get the counter usedSize afs_used (active file system used)! Can you check which template Harvest is using for OCUM? And that afs-used is in the list of volume counters in the template?
Btw, afs_used_percent is exactly what you are looking for, but this is calculated internally by Harvest, so if for some strange reason it's not getting afs-used from OCUM, then this counter shoud also be incorrect.
Don't worry line 2591 in worker, it is completely unrelated to this case (also it's the most tested line in 1.6, so it should be fine :)).