Checking space required on tape

Hey All..

I am looking into getting a BUE Remote Agent for Windows server to have a Win2k8 server backup to tape. This may sound simple or dumb but how do I find out if I have enough storage space left on the tape? 

When I look at the byte count on the Job Monitor it states in the region of  1,550,400,789,647 but if I were to do a right click on Media Label > Properties > Statistics tab, I can see total cap = 745GB / used cap = 402GB / available cap = 343 GB with compression ratio 3.63:1. So which one should I look at?

I am using HP 1.6TB LTO 4 (C7974A) tapes at the moment hence I just wanted to know if that's sufficient space to have add another server to the tape or do I need bigger space (3TB LTO5 etc) to accomodate?

Thanks in advance.