I believe that you can simply add up the pressure drops from each component in your system (as long as they are in series)
have a look at
http://forums.procooling.com/vbb/sho...&threadid=5068
It will show the relationship between pressure an flow for many pumps
If you have an eheim 1048 and the total pressure drop through your system is 5 feet then the flow will be zero.
If the pressure drop in your system is about 3 feet then your flow will be 100 GPM
(at least that is what seems to make sense to me)
I hope this helps