Question : Problem: LTO Tape Compression

I've read online a 400GB LTO tape w/800GB compression can expect about 600+GB of compression space.  How is this calculated and how much actual compression can I expect from other various sized LTO's? and again how would that be calculated? or would it really be from experience?

Answer : Problem: LTO Tape Compression

You can not calculate it.

The compression ratio given is a theoretical typical value set by the marketing heads.

In real life the compression ratio is going to depend upon the data itself. If for example you backup compressed data such as zip, mp3, jpg or any other type of compressed data there will be no compression. If you backup binary type data such as exe, dll, iso, ghost type images there will be very little compression. On the other hand some data like text files will typically have very high compression ratios.