Questions about compress table

[Kathey Marsden]

http://db.apache.org/derby/docs/dev/ref/rrefaltertablecompress.html says 
a non sequential compress uses "an amount equal to approximately two 
times the used space plus the unused, allocated space"
and it says SEQUENTIAL uses less space.  How much space does SEQUENTIAL use?
Also is there a general guideline for how much longer SEQUENTIAL will take?

Thanks

Kathey