There is no display for that in RAIDz*, it depends on the ashift, zvolblocksize, compressability and number drives in a vdev. As with a lot of CoW filesystem, you should not excees 80% usage due to performance and will not excees 93%, then is your pool full.So where actual shows what I can use?
If you want better estimates for the size, just use stripped mirrors (raid10 and it is "see what you get" ) or don't use ZFS.