Replica volume

For files:

• (Data source size x 3) / 2

For Exchange data:

• Data source size x (1 + log change) / (alert threshold – .05)

For SQL Server data:

• Data source size x (1 + log change) / (alert threshold – .05)

For Windows SharePoint Services data:

• Total size of all databases/ (alert threshold – .05)

For Virtual Server data:

• Data source size x 1.5

For system state:

• (Data source size x 3) / 2

For Hyper-V

• Data source size x 1.5

http://technet.microsoft.com/en-us/library/ff399599.aspx