Exchange sizing

I'd like to have some clues about DPM sizing for Exchange.

http://technet.microsoft.com/en-us/library/hh758141.aspx

For Exchange data:

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

is it 90%-5? 85% of the db size?

• 4.0 x retention range in days x log change x data source size + 1600 MB

i don't understand these calculations.

is there anybody who can give me some clues about?

thank you