Hi there,
First off, LTO-3 has a 400GB NATIVE capacity and is often seen as 800GB capacity but that assumes 2:1 compression - it's there on the label.
Now to the question regarding tape size - DP does not care about such things with DLT or LTO tapes and you can tell DP to format a tape to pretty much any size you would like - most people just leave it to the default of 100GB I think. What DP does use is the end of tape marker - when it see this, it knows it has reached the end of tape and will request another tape if the backup has not finished.
I have never used the "pool" capacity figures or graphs becuase you can never determine the exact compresion ratio you will get when backing up your data - the only figure I trust is the "Used" amount.
There is one cavet to this of course - you setup DP to use a non-compression device - this way DP will still wait for the end of tape marker and you can format the tape to the true NATIVE capacity with corect reporting in the pool stats.
If you are only getting 400GB on a tape, then I assume you are using a non-compression device (does it end with a "N"?)
Just to confuse matters even more, if the performance of the backup is less than that required from the tape drive, then you will use up much more tape to store data - I've seen >60% reduction in tape capacity because of bad performance.
HTH
Mark