I'd like to know the exact math of how to calculate usable capacity on a NetApp filer's aggregates. Out of experience I've been using a magic factor of 0.65-0,7 times the net RG capacity of all the aggregate's RGs.

Just as a simple example: 3 shelfs, each with 24 1TB spindles with 3 spares form an aggregate with RG-size 15, ie 3 RGs, one plex, total 45 TB capacity. The usable capacity from the RGs is ((15-2)*3) = 39TB. There are no volumes on this aggregate and the aggregate snap reserve is 5%.

The system reports a usable capacity of 27TB on that aggregate which is pretty much 39TB times the magical factor. Can anyone provide some insight?