here's a site for calculating single pressure drop in pipes of certain diameter, i'm not sure how much it really applies, but here it is:
http://www.processassociates.com/process/fluid/dp_1.htm
and a "general" idea of some common pressure drops for radiators might be obtained from an article by BillA at overclockers.com not sure of the name, maybe someone else can provide a link.
of course if you are using a non-conventional rad. it might not be too helpful