You will never be able to precisely determine the amount of free Space on a Media.
Everything DataProtector can give you is an estimate compared to the first full write on the Tape.
Amount writeable to a Tape depends on the Data-Structure and how well it compresses in Hardware.
So LTO3 can hold 800GB (or less) with already compressed Data and over 2TB with good compressable Data.
The Graphics will only update once you have written the Tape from start to end once, after that DP uses the amount written during that pass as estimate for future writes.
To determine how much is free in your pools you should devide the Media into Retention Groups or according to DataType (DB, Archvies, Logs, VMs, File-Data, etc). After some cycles you can then calculate a average Media Size/Usage over each Pool. Will never be a precise value as you would have on a FileLibrary.